which ones are good?

Utopia is the best DLC; it adds a lot of impactful content that can define entire playstyles. After that it's a matter of preference and it depends what you want to add to your game.

The only DLC's I'm not a huge fan of are the species packs. They are mostly cosmetic (entirely cosmetic in the case of humanoids and plantoids) and the base game is already loaded with a wide variety of cosmetic options so I don't really feel the need for more personally.

Thanks for the reply! There is so much DLC for this game, and I didn't want to spend a lot of money.

THere is a nice list of all the DLCs with brief estimations:
https://www.pcgamesn.com/stellaris/best-stellaris-dlc

I've read the article a bit, and it looks to be somewhat outdated. It seems like they add new DLCs to it but do not revise the entries for the previous ones, making them less useful. For example, the article says that the Utopia DLC is good in part do to accession perks but I'm pretty certain that nowadays they are a part of the base game. Or it mentions as a downside of the Synthetic Dawn DLC that you have to build robot pops manually but that is not the case in the current pop system.

Well, the article is not very deep anyway, but useful for a quick overview.
Also, the new features of DLCs are ususally incorporated into the base game via the game patches.

There are some youtubers who have ranked the DLCs, e. g. ASpec:
https://www.youtube.com/watch?v=DQg8am028ko (pre-Federations)
https://www.youtube.com/watch?v=riqizg2V8Q8 (about Federations)
He has also videos about the particular expansions.

And for the sake of completeness, here is ASpec's take on Nemesis and the patch:
https://www.youtube.com/watch?v=wRnxkqHY6p4

Thanks for all the info. It is very helpful.