Tasha's Cauldron of Everything introduced some of my favorite spells in 5E. While there had been plenty of "Conjure X" spells, which pulled (usually multiple) creatures from the Monster Manual out and put them into battle (sometimes with really insane consequences, like allowing you to summon 8 Pixies that can each cast Polymorph once a day) these spells created a consistent, single, powerful minion that used your spell attack modifier.
And each works the same way - getting a number of attacks equal to half the spell's level rounded down (the lowest-level one of them is the 2nd level spell Summon Beast, so no issues with having zero attacks) and getting a damage boost on the attacks' damage for the spell's level.
The consistency does let you, to a large extent, pick your favorite for thematic reasons. A classic Merlin-style Wizard might go for Summon Elemental or Summon Construct, while a darker spellcaster might go Summon Undead or Summon Fiend.
Wizards and I want to say Warlocks gain access to the most options here, but Artificers, Bards, Druids, Clerics, Paladins and Rangers get some as well (Sorcerer subclasses often get one, but the base class, I believe, doesn't get any).
5.5 saw the old Conjure spells significantly changed to be more like ongoing spell effects (including the insanely powerful Conjure Minor Elementals,) with an intent to make these the primary "I put more creatures onto the battlefield" spells. Now, there are actually a handful of additional spells that can do things like this. Giant Insect is actually essentially another one of these spells. In contrast, though, Animate Dead and Create Undead are much more like their 5.0 versions - the real distinction being that the monsters don't despawn when the spell ends; you just lose control of them.
One spell from 5.0 didn't make the transition to 5.5, and while it should work just fine, I'm going to leave it out in our comparison here. Summon Shadowspawn is actually very conceptually cool, and I love having non-undead monsters for the Shadowfell. Eh, maybe we'll cover it at the end of this.
I'm going to compare the various summoned spirits by three primary criteria: damage output, survivability, and utility.
To make a fair comparison, we'll consider these when cast at 6th level, which is the highest base level for any of them (Summon Fiend). Notably, Warlocks don't actually get to cast any but Summon Fiend at 6th level because of how Pact Magic works (they can pick up Summon Fiend as their 6th level Mystic Arcanum, though they also can't upcast it to 8th level). Naturally, Artificers, Paladins, and Rangers can't cast these at 6th level (unless they multiclass with a pure spellcaster) but every spell should be available to at least one pure spellcaster.
Thus, each will get three attacks, and we'll total the damage output along those lines. Now, there's a subtle nuance - some creatures use a higher damage die, which means that crits will be a little more powerful even if they have the same average damage as another, though this is a pretty subtle difference.
A summoned spirit's AC and HP tend to scale with their spell level, and again, we'll list the 6th level version. Damage can also vary on the form that the summoned spirit takes, so we'll list each version.
Giant Insect:
While not called "Summon Giant Insect," as mentioned before, this effectively works just like one of these spells, with two major differences: first, it has no material component, meaning that you don't need to buy the (actually pretty costly) components that the rest of these need. However, it only lasts 10 minutes, meaning you might not be able to get quite as much use out of it.
Damage: 1d6+9+1d4, or 15 (mix of piercing and poison) per hit, so 45 total. Alternatively, 1d10+9, or 14.5 (bludgeoning), so 43.5 total.
Defensive: AC 17, HP 50
Utility: The insect in Spider form (yes, I know, it's an arachnid - the spell should be Giant Bug) can reduce a creature's movement speed to 0 on a hit, and this attack is even ranged. In Centipede form, it can attempt to poison a nearby target as a bonus action. Wasp form can fly.
Summon Aberration: (Warlock, Wizard)
Damage: 1d10+9 (slashing), or 14.5, for 43.5 total, or 1d8+9 (psychic), or 13.5, for 40.5 total. (Potentially 2d6, or 7, AoE psychic damage)
Defensive: AC 17, HP 60
Utility: Slaad has regeneration and also prevents healing with its slashing attack. Beholderkin can fly with hover and has ranged attacks. Mind Flayer has AoE aura (save for none, can do friendly fire).
Summon Beast: (Druid, Ranger)
Damage: 1d8+10 (piercing), or 14.5, for 43.5 total.
Defensive: AC 17, HP 40 (air) or 50 (land or water)
Utility: Land has a climb speed, water has a swim speed, air has a 60 foot fly speed. Air has flyby, Water has Water Breathing (not amphibious) and Land and Water have Pack Tactics.
Summon Celestial: (Cleric, Paladin)
Damage: 1d10+9 (radiant) or 14.5, for 43.5 total, or 2d6+8 (radiant) or 15, for 45 total.
Defensive: AC 17, or 19 (defender), HP 60
Utility: Radiant resistance, 40 foot fly speed. Has a 1/day healing touch for 2d8+6 (15 average). Defender grants 1d10 temp HP to itself or a nearby friend on hit with its attack.
Summon Construct: (Artificer, Wizard)
Damage: 1d8+10 (bludgeoning) or 14.5, for 43.5 total.
Defensive: AC 19, HP 70
Utility: Resistant to poison, does 1d10 damage to attackers or grapplers (Metal), slows and prevent opportunity attacks to nearby creatures (can hit friends) on a failed wisdom save (Stone) or gets reaction attacks or free movement as a reaction when taking damage (Clay)
Summon Dragon: (Wizard)
Damage: 1d6+10 (piercing) or 13.5, for 40.5, plus AoE cone for 2d6 (7) of draconic elemental damage.
Defensive: AC 20, HP 60
Utility: Swim speed, 60 foot fly speed, resistance to Acid, Cold, Fire, Lightning, and Poison damage, Blindsight, shares one of its resistances with you (chosen at summoning).
Summon Elemental: (Druid, Ranger, Wizard)
Damage: 1d10+10 (bludgeoning, cold, lightning, or fire, depending on element,) or 15.5, for 46.5 total.
Defensive: AC 17 HP 70
Utility: 40 foot speed, with Burrow (Earth) Fly with hover (Air) and Swim (Water). Amorphous Form (Air, Fire, Water). Immunity to fire damage (Fire) or resistance to Lighting and Thunder (Air), Acid (Water) or Piercing and Slashing (Earth). Full immunity to poison.
Summon Fey: (Druid, Ranger, Warlock, Wizard)
Damage: 2d6+9 (Force) or 16, for 48 total.
Defensive: AC 18, HP 60
Utility: Fly speed, immune to charmed, bonus action with 30-foot teleport and secondary effects: spirit gains advantage on next attack (Fuming,) charms a nearby creature on failed wisdom save, both you and fey spirit charm the target (Mirthful) or creates 10-foot cube of magical darkness (Tricksy).
Summon Fiend: (Warlock, Wizard)
Damage: 1d12+9 (necrotic) or 15.5, for 46.5 total, or 1d8+9 (slashing) or 13.5 for 40.5 total, or 2d6+9 (fire) or 16, for 48 total.
Defensive: AC 18, HP 50 (demon,) 40 (devil,) or 60 (yugoloth)
Utility: 40 foot movement speed, climb speed (demon) or 60 foot fly speed (devil). Yugoloth teleports 30 feet with each attack. Magic Resistance and resistance to fire. Devil has devil's sight. Demon explodes when reduced to 0 HP, doing 2d10 fire (11 average) in a 10-foot emanation (dex save for half). (Can cause friendly fire). Devil's attacks are ranged.
Summon Undead: (Warlock, Wizard)
Damage: 1d8+9 (necrotic) or 13.5, for 40.5, or 2d4+9 (necrotic) for 14, or 42, or 1d6+9 (slashing) or 12.5 for 37.5.
Defensive: AC 17, HP 60 (ghostly or putrid) or 50 (skeletal)
Utility: Ghostly gets 40 foot fly speed with hover. Immunity to necrotic and poison damage. Ghostly has incorporeal passage. Putrid has emanation that poisons on a failed con save, and attacks that paralyze a poisoned target. Skeletal has ranged attacks. Ghostly frightens on hit.
Conclusions and Results:
In terms of damage output, the Devil version of Summon Fiend and the Fey spirit both wind up doing the most. The devil can fly and its attacks are ranged, allowing it to stay out of the melee, though the Fey's force damage will basically always do it to whatever you're attacking.
Defenses are fairly consistent, though the dragon gets the highest AC of them all, with runners-up being the Guardian form of the Celestial and the Construct. For HP, the Elemental is the big winner, and is the only one that scales up 15 HP per spell slot, rather than 10.
For utility, this is a lot harder to evaluate, and depends on your situation. The Undead Spirit has some very powerful utility (especially if you have other ways of getting a target poisoned).
I think it's notable that the Aberration, Fiend, and Undead all have ranged options, which can be very good if you want to just use them as a distant turret. Of these, the Aberration and Fiend can also fly, which allows them to not only be distant but truly out of reach of many monsters.
Another potential thing to consider is size: the Celestial, Dragon, and Fiend are all Large, which means you could ride on them, and the former two have fly speeds while the latter has one form with a fly speed. Technically they go after your turn in combat, but if your DM is cool, they might let you have the spirit act simultaneously with you to function as a kind of uncontrolled-controlled mount. (Oh, also the Giant Insect, lest I forget.)
All this being said, though, I really think you can pretty confidently pick whichever of these you think seems most in-theme with your character. Even the ones that don't do quite as much damage, like Summon Beast, are still nearly doing as much when upcast.