So, I'm in a kind of middle ground between power-gaming min-maxxers and "vibes, man" character building in D&D. I like my characters to be powerful, because that's the fantasy of the game as far as I'm concerned. I don't do enough one-shots to actually get a chance to try out every whacky out-there build or joke character, so for a long campaign, I want a fleshed out character whom my fellow players will learn to love (or fear, as was the case with by Warlock in Descent into Avernus - very much a "glad he's on our side" kind of character) but who is also just mechanically working within expected parameters.
Broadly, the primary thing that I'm really skeptical toward is multiclassing. I think that there are for sure cases where a multiclass build makes sense for a character. In our Wildemount game, one of the two Paladins multiclassed into Warlock and it made perfect sense - the entire campaign, he's been haunted by the charred ghosts of his former comrades who were all killed in a magical firestorm at Blightshore (one of the major "don't go here" places in the Exandria setting,) and by accepting their direct aid, he's basically made a pact with them (though curiously, it seems to be a Great Old One patron, rather than Undead, as I'd have assumed. I don't really know what is going on with them, so all I do is mutter "got a light?" when the ghosts appear to the rest of the table).
Still, I generally try to build characters along a single class track unless there's a really compelling story reason. The only truly mechanical reason I'd do it is if I wanted to play a Pact of the Blade Warlock, because I think they get so, so many of their problems solved by just starting as a Fighter for their first level.
In that campaign, I've been playing a Wizard. I really can't complain at all about the class, as I've felt plenty powerful with the spells I've picked. Fireball is great unless you roll really awfully low, and even then, even if you're only getting like 14 damage out of it (which is only half of its average damage) it's still over a big enough area that that feels pretty significant if you're in a target-rich environment.
My usual go-to concentration spell is Summon Undead (while my character is a very classically heroic, quite naive young wizard who was basically just out of wizard school at the start of the campaign, his favorite professor was the Necromancy one, and so he likes summoning forth undead spirits to help,) which I think is a very worthwhile option, and if you can maintain concentration, a pretty spell-slot efficient one at that. It's also fairly versatile, with various forms of crowd control or just straight, safe ranged damage (in a fight against a bunch of humans working with a succubus, I was using Drow Poison on a Light Crossbow with True Strike, which was often setting up my Putrid Spirit to paralyze them - after I had knocked the fiend unconscious with the poison on the first turn of the entire combat, which he remained for like three rounds).
But while the Summon spells are great for sustained single target damage, I think it's pretty clear that there are ways to do bigger bursts of damage.
Conjure Minor Elementals, which in 2014 was mainly there to let you summon a bunch of Mephits or maybe an Azer or two, is profoundly different than it used to be. I've written about this spell many times before.
The "obvious" path to getting a lot of power with this spell is to use it on a character who is a melee/martial hybrid - someone who can get a lot of attacks in and thus multiply the 2d8 bonus the spell grants to your attacks as much as possible.
While I think that it's plenty powerful on a tier 4 Eldritch Knight, who get 4th level spells at level 17 and will eventually get four attacks per attack action at level 20, I think the damage winds up scaling up better with pure spellcasters. Bladesingers and Valor Bards are going to naturally want to use this (Dance Bards too, I guess). Bards, of course, will need to wait to get Magical Secrets to get this spell, as it's not natively on their list. The spell is actually only on the Druid and Wizard spell lists, though there are some subclasses (and I know for sure one or two of the Eberron Dragonmark spells of the mark) that will get it as well.
An Eldritch Knight will be able to make more attacks than a Bladesinger using this spell, but won't be able to upcast it. And with just one or two levels of upcasting, that benefit begins to fall off.
Druids, of course, can also build around weapon attacks - you can get martial weapon proficiency with the Warden Primal Order option, or you can just shape-shift, with Moon Druids in particular getting some forms that will be able to attack three times a turn.
But let's say that you really just want to stick to spells? What are the best options?
Well, the answer is pretty clear: you want non-concentration spells in which you (not a pet) are making lots of attacks. These attacks do need to be within a 10-foot range (though because my Wizard is a Scribes Wizard, I think I should be able to use my Manifest Mind to cast CME and move my floating book-spirit around while I pelt away from a safe distance and still get the damage bonus.
So, what spells?
The only cantrip I'm aware of that does multiple attacks is Eldritch Blast, so if you can get this spell on a Warlock, it'll be very good. But we're looking at the classes that get this spell natively. An attack cantrip will certainly benefit from the spell, but you're only really adding 9 damage a turn on a hit. Not really what you want for a fourth level spell (the skeletal spirit from Summon Undead is going to be hitting for like 12 twice a turn cast at 4th level).
The only Druid spells I could find that aren't concentration but call for attack rolls are Elminster's Effulgent Spheres and Syluné's Viper, each of which only allow one attack as a bonus action. This could mean, coupled with a cantrip like Starry Wisp, you could get two attacks out of this, but that doesn't seem all that exciting.
Wizards, however, I think, will feast on this:
Chromatic Orb could hit multiple times, though you'd need to have all your enemies packed tightly within your 10-foot emanation to get the benefit of the CME.
Scorching Ray is probably your first, easily accessible, really powerful option here. At its base 2nd level, Scorching Ray shoots three bolts that each do 2d6 damage, meaning that with the 2d8 bonus from CME, you're already more than doubling the damage you do with the spell. Upcasting Scorching Ray is going to add additional bolts, rather than making each bolt more powerful, which is great, because it's a further multiplier of your damage. Casting CME at 4th level and Scorching Ray at 3rd, you'll be getting 4 bolts that are each hitting for about 16 damage apiece. If they all hit, that's 64 damage.
Steel Wind Strike is the next option. This will effectively extend the range of your CME because you'll be teleporting around for each strike, and your emanation travels with you. The trick is that this only gets full damage if you can hit all five targets - it has to be spread out, not concentrated on one target like Scorching Ray can. Still, each hit will land for 6d10 (33 on average). With 2d8 added to each of those, you'll be hitting for an average of 41 each. If you can somehow also get advantage, that'll be great, because all the damage is doubled on a crit. To compare, if we were casting Scorching Ray at 5th level with a 4th level CME, we'd be getting 6 rays each dealing 2d6+2d8 (again, 16 average) and so in theory we could do 96 damage total. Against 5 targets, of course, Steel Wind Strike is doing a total of 205, but we can't concentrate it on a single target. (Also, SWS doesn't upcast). Incidentally, if were were to flip that Scorching Ray option, casting CME at 5th and SR at 4th, we're looking at 5 rays, but each is now dealing 2d6+3d8, or about 20.5 on average, for 102.5 total. So, yeah, I think if you want to build around this, you probably want to use an upcast Scorching Ray for single targets and Steel Wind Strike for situations with 3-5 targets.
One spell that you might not initially think of (I have to credit Colby over a D4: D&D Deep Dive for this one) as working with these is Contingency. This 6th level spell lets you effectively pre-cast a 1st-to-5th level spell at an earlier point that will take effect when its trigger occurs. This could be something like casting Water Breathing when you're fully submerged (not necessary for my Triton, but I get it) but in this case, it's less of an emergency than an opportunity to cheat the action economy: if you come up with a special word or phrase as the trigger, you can cast this the night (or some days) before you need it so that you can get CME running and still have your action and the ability to cast a spell with a spell slot on your first turn. For as insane as the damage of CME is, you always need to put a little asterisk there that you still typically wind up doing no damage on the first round of a fight, and so if the fight only goes two rounds, the insane damage output of these spell is effectively halved. But if you set up a contingency, you will (at least for one major fight) be able to hit the ground running.
Like Druids, Wizards get access to the aforementioned Elminster's Effulgent Spheres and Syluné's Viper, but these aren't going to be really maximal, damage-wise.
Spellfire Flare (also from Heroes of Faerun,) however, works great with this. Like Scorching Ray, it's a low level spell that adds additional bolts when upcast rather than just adding damage to the attack. Each attack does 2d10 Radiant damage, and at base 1st level, it does just one bolt. Targets won't benefit from half or three-quarters cover against it.
Ok, so how is it, damage-wise? Do we replace Scorching Ray with this?
The problem is that it does fewer bolts - when upcast to 2nd level (Scorching Ray's base level) it will do 2 bolts, which means that it's always one fewer shot than Scorching Ray. On the other hand, the actual bolts hit harder, for 2d10 (11) rather than 2d6 (7). At 2nd level, in fact, without CME, it's already slightly outpacing Scorching Ray with 22 average damage compared to 21.
But Scorching Ray has an edge on number of bolts. So, if we toss in a base level CME, and then cast, say, a 4th level version of each of these, what are we looking at? Scorching Ray is doing 5 bolts for 2d6+2d8 each, so 80 damage. Spellfire Flare is doing 4 bolts for 2d10+2d8 each (20 apiece) so... 80 damage. Huh. Didn't expect to hit the level at which they were totally equal. And I guess further upcasting favors Spellfire Flare because Scorching Ray is adding 16 while Spellfire Flare is adding 20 with each further upcast.
However, if we upcast CME as well, the balance tilts toward Scorching Ray. With a 5th level CME and 4th level attack spells, we're looking at 2d6+3d8 with 5 bolts at 4th level (102.5 total) versus just 4 bolts of 2d10+3d8 (24.5 each) for a total of 98.
Honestly, I think the choice between Spellfire Flare and Scorching Ray is really a bit of a judgment call. If you're upcasting CME, Scorching Ray does better, and likewise if you're not upcasting Scorching Ray or Spellfire Flare very high.
However, Spellfire Flare has two other factors that really help it: it's Radiant damage, which is far less often resisted than Fire damage (and lots of monsters are fully immune - again, as a Scribes wizard this is less important to me) but also, the ability to ignore partial cover means, situationally, you're going to be hitting more often. Truly, both are strong options, and I think it mainly depends on how much upcasting you're doing and with which spells.
Crown of Stars has the same issue as the Effulgent Spheres and Viper spells - while sustainable, you only get one attack per turn. That attack does do 4d12, but you're only increasing that 26 average by 9 with CME each turn.
And yeah, that's about it, as far as I can tell.
To review:
I think your best options for maximizing the power of Conjure Minor Elementals with spells alone are Scorching Ray, Spellfire Flare, and Steel Wind Strike.
The latter does a lot of damage on its own, but must be treated as an AoE option - you can't concentrate its power on a single target. Scorching Ray and Spellfire Flare are both really solid options, but you might need a spreadsheet to figure out which is optimal in which circumstance, as the sliding scale of upcasting will change which comes out on top. You can, of course, pick and prepare both, but I think you'll probably just want to pick one or the other.
Remember that this is for burst damage - to maintain this amount of damage round after round, you're going to need to expend a lot of spell slots. But if you really want to melt a boss quickly, it's a good way to go.