Shadow of moil provides at least 48 million HP
First, a closer look at Grim Harvest:
Once per turn when you kill one or more creatures with a spell of 1st level or higher, you regain hit points [...]
On each creature's turn, you have a chance to regain HP with Grim Harvest (including your own). The trick will be dealing damage on each enemy's turn. For that, we turn to the 4th-level necromancy spell shadow of moil (XGtE, p. 164):
whenever a creature within 10 feet of you hits you with an attack, the shadows lash out at that creature, dealing it 2d8 necrotic damage.
Assuming (as your question states) that one creature dies each turn, shadow of moil will provide infinite HP. There is no limit to the number of turns per round. You'll need to multiclass warlock (or bard), since shadow of moil is warlock-only.
Even a non-infinite amount of creatures will provide a very large amount of HP. I'll use quicklings (XGtE, p. 187), since they're really fast. I'll also assume that every attack hits, and every quickling is one-shotted by shadow of moil, which means that we don't have to worry about moving them after they attack. (Body disposal is left as an exercise for the reader.)
In order for shadow of moil to trigger, a quickling needs to attack from 10 feet or closer with its ranged/melee dagger attack. With a speed of 120 feet, a quickling will need to start within 130 feet of you to get into range.
Let's assume we're playing on a grid, which turns the circle of quickling attackers into a square. That square is 53 grid squares (265 feet) on a side, totalling 2 808 squares, if we leave out the one you're standing in. Since quicklings are tiny, four can fit in one square, meaning that 11 232 quicklings are able to attack in the first round.
Necromancer 7 / Bard 10 / Sorcerer 3 gets you Grim Harvest, shadow of moil, and Extended Spell. You upcast shadow of moil to 9th level (and apply Extended Spell), granting you 27 HP from each quickling, for a total of 303 264 HP in the first round. More quicklings will have no trouble keeping up the pace (by following their ill-fated brethren towards you), giving you a total of 6 065 280 HP in 2 minutes.
That's just using the ground. Next, you can fill the skies above you with owls. An owl needs to start within 60 feet of you to be able to attack, which leaves a 27 x 27 x 13.5 grid square area above you where owls may start. (The 0.5 comes from the half-square above the heads of the quicklings on the ground.) Owls are tiny and can fly, so 8 of them fit in each square. A total of 78 732 owls can attack in the first round. After 2 minutes, the horde of owls and quicklings combine to grant you 48 580 560 HP.
Prismatic wall (an abjuration spell) pushes that...further
If you're willing to use spells from any school, long duration damage spells are the way to go. (Thanks to Someone_Evil in the comments!)
A creature takes (a lot of) damage just from touching a prismatic wall, so creatures can now just Dash into it. There's nothing preventing the wall from floating in midair, so we'll use the 90 foot by 30 foot version high in the sky. Surrounded by owls, naturally.
Owls can reach the wall from 120 feet away in any direction, which gives us 66 x 54 x 48 grid squares of owls. Total owls: 1 368 576. We'll skip to the punch line here: 4 926 873 600 HP in 20 minutes.
Returning to reality
Alright, we've had our fun, but what about practical uses? Here are some good wizard spells for damage outside of your turn (the fact that none of these are necromancy spells is disappointing):
- Evard's black tentacles  - mix of damage and control; save for no damage, though.
- Sickening radiance  - nice large area; save for no damage, though.
- Wall of fire  - easy to place (without interfering with allies).
- Dawn  - big area, decent damage, and you can reposition it.
- Maddening darkness  - massive area.
- Prismatic wall  - big area, big damage, doesn't interfere with allies, non-concentration, blinds foes, and doesn't care about antimagic field. What a fantastic spell!
Any clerics looking to take 2 levels of wizard should strongly consider necromancy. Spirit guardians is practically begging to be used with Grim Harvest, and it's already a great spell that doesn't interfere with allies.