Sleep is by far the best first level offensive spell in the PHB for use against a single target.
Not really.
On a failed save,...
Sleep doesn't allow a save.
...and assuming 5d8>their HP...
That's 22.5 HP on average at level 1. I'll admit I haven't taken a good look at the DMG or the Monster Manual yet, are there a lot of sub-CR2 enemies with more HP than that?
Using a bigger spell slot gets you an average +9 HP per additional level of the slot used, but since you gain higher level slots more slowly than you gain levels that's not going to keep up with the bigger HD creatures too well. Probably better tactics after level 1 to start whittling enemies down a bit before dropping a Sleep-nuke on them. Following up a big evocation spell with Sleep is more effective than casting the same evocation twice in most cases.
...and they are not Undead or immune to Charm,...
Or elves, half-elves, constructs, or anything else that doesn't sleep. That's definitely one of the big weaknesses of Sleep, which I mentioned in my post.
...Sleep renders them unconscious for ten rounds. At that point you can starting running away from the encounter with a high chance of success, which flight or mounts, or fast running speeds can already do..... Oh you meant offensively. Well you get one double-dice attack against it then it just wakes up.
Or, y'know, you could be smart and have your party gather around to attack them all at once. Four double-dice attacks are bound to be more effective than one, especially if the whole party novas.
You could also take advantage of the fact that unconscious creatures auto-fail Str and Dex saves. I'm sure there are various ways to restrain or immobilize a target that could prove effective if you don't think your team can take them out with a round of auto-crit curbstoping. Again, it's generally better to combo Sleep with another spell than end up casting that other spell multiple times waiting for your enemy to fail a save or for the damage to stack up.
...Which means it really just deals 8d10 (44) damage if you follow up with a Cantrip, which is only two points higher than a Warlock using Antagonizing Blast...
Well, it could be as much as 8d12 (54 avg) with a cantrip if you use Poison Spray, but Warlocks can
also take Sleep as one of their spells if they go with the Archfey patron. I believe the average crit damage for an Agonizing Eldritch Blast is 84 (16+5 x 4 beams), assuming max 20 Cha and level 17+. Thought I recalled seeing something around here about an errata nerfing crits with multi-beam spells, but it isn't in the latest errata doc. In any event each beam would crit since they all hit at once and attacks auto-crit unconscious targets.
...except his is Saveless and works against any type of Creature of any CR or remaining HP.
Eldritch Blast requires an attack roll, as does Firebolt and most other damaging cantrips. You get advantage on attack rolls against unconscious targets within 5ft, but you don't auto
hit. Create Bonfire is a dex save cantrip that deals 8d8 on a crit at lvl 17+, Sacred Flame is does this as well, and Acid Splash is dex save or take 8d6 on a crit, and can hit two adjacent targets.
and with a 9th level slot it comes in just below Power Word Kill on average (94.5 HP vs 100)--though it's max damage is much higher. (168 HP)
Or you could use a 7th level Slot to banish them to one of the elemental planes they won't survive on. Heck choose the Swamp of Oblivion and they can't return for 100 years so unless you're an elf and your target is another such long lived creature problem solved.
Which spell would this be?
Banishment is a 4th level spell than can only send it's target to it's home plane or a harmless demiplane. It also allows a save which Sleep does not. I'm not finding a 7th level spell that does what you describe--although in the interest of full disclosure a 6th level Disintegrate is one of the few damaging spells that has better one-shot capability than Sleep cast with an equivalent level slot.
A 6th level Sleep affects 15d8 HP (Min: 15 Avr: 67.5 Max: 120), Disinregrate does 10d6+40 (Min: 50 Avr: 75 Max: 100), though only on a failed Dex save. On a successful save disintegrate does nothing, making it a better candidate for mop-up duty after Sleep has knocked your foe unconscious.
Better to test the target's reflex DC with a low level spell or a cantrip on the first round while letting your allies bring down the target's HP a bit. If you're really confident they'll fail, give Disintegrate a try. If not, dropping a 6th level sleep on them first is better than casting Disintegrate twice and potentially having them make the save both times.