So I currently have the 2014 version of cloudkill, but it doesn't work as I thought it did. It doesn't do damage as soon as it's cast. One person seems to think the 2024 version does. Is that also how other people understand it? I'm not that happy with the spell, so the question for me is, if I ask my dm if we can just treat my version as the 2024 version (I can't switch as it's tied to my subclass). I'm just stuck with it, or I ask if I can pick another spell. The others I have looked at are:

Hunger of Hadar (2024 version)

Jallarzi's Storm of Radiance 

Sickening Radiance

Synaptic static

I'm playing a level 11 warlock with a dc of 19. We seem to be just fighting undead or corrupted creators now. Which does people think work best

the new version does damage when cast and does damage if it engulfs a creature when it moves, it's damage triggers more often in general.

What your DM allows is down to your DM, you can always ask.

Something to consider is that Cloudkill (both the 2014 and 2024 versions) deals Poison damage exclusively, and a lot of Undead, even low-level ones, are just completely immune to that.

Synaptic Static may be a better choice because it deals Psychic damage (which is pretty rare to find something even resistant to, let alone immune to) and it requires an Intelligence save (which Undead are often not particularly good at). The drawback is that it's an instantaneous effect and a single casting will only hit any given creature once.

If you want something with a longer effect, Hunger of Hadar is decent. It won't match the damage potential of Cloudkill, even cast at level 5, but it'll work on a lot of things that Cloudkill won't.

As @R3sistance explained, spells like Spirit Guardians, Moonbeam, Conjure Animals or Cloudkill and others with similar wording work differently between 2014 and 2024.

This thread has several scenarios comparing the 2014 and 2024 behavior for Moonbeam: Question about the new rules of when creatures take damage from ongoing spell effects - Rules & Game Mechanics

For the 2014 version, there is a SAC entry to explain it, because the wording was confusing for some people: 

Does moonbeam deal damage when you cast it? What about when its effect moves onto a creature?

The answer to both questions is no. Here’s some elaboration on that answer.

Some spells and other game features create an area of effect that does something when a creature enters that area for the first time on a turn or when a creature starts its turn in that area. On the turn when you cast such a spell, you’re primarily setting up hurt for your foes on later turns. Moonbeam, for example, creates a beam of light that can damage a creature who enters the beam or who starts its turn in the beam.

Here are some spells with the same timing as moonbeam for their areas of effect: blade barrier cloudkill cloud of daggers Evard’s black tentacles forbiddance moonbeam sleet storm spirit guardians

Reading the description of any of those spells, you might wonder whether a creature is considered to be entering the spell’s area of effect if the area is created on the creature’s space. And if the area of effect can be moved—as the beam of moonbeam can—does moving it into a creature’s space count as the creature entering the area? Our design intent for such spells is this: a creature enters the area of effect when the creature passes into it. Creating the area of effect on the creature or moving it onto the creature doesn’t count. If the creature is still in the area at the start of its turn, it is subjected to the area’s effect.

Entering such an area of effect needn’t be voluntary, unless a spell says otherwise. You can, therefore, hurl a creature into the area with a spell like thunderwave. We consider that clever play, not an imbalance, so hurl away! Keep in mind, however, that a creature is subjected to such an area of effect only the first time it enters the area on a turn. You can’t move a creature in and out of it to damage it over and over again on the same turn.

In summary, a spell like moonbeam affects a creature when the creature passes into the spell’s area of effect and when the creature starts its turn there. You’re essentially creating a hazard on the battlefield.

Jallarzi's Storm of Radiance is probably your best bet out of all of those, it deals thunder & radiant damage that are very rarely resisted, it also makes enemies blinded without a save (giving you and all your allies Adv on attacking them) and prevents spellcasters from casting spells again without a save (which can force enemies to take AoO). However, since it is smaller and doesn't move, it is best in a party with multiple allies who can push enemies into it. 

However, if area targetted is more important to you (e.g. often fighting off hordes of weak undead) then Synaptic Static is better despite not being persistent.