Species packs are mostly cosmetic and unless you want more diversity, aim to get them when discounted.
The main DLC are:
Leviathons, adds big bosses to the galaxy and are fun until you’ve beaten them all. But it also adds the End Game event “War In Heaven” which is good fun and well worth getting.
Utopia, adds megastructures and ascension perks which caused a bit of an issue because a lot of those perks then got put into the main game even for those without the DLC. Not essential, but worth getting on sale.
Apocolypse, adds titans which are super huge capital ships and death stars which can block or destroy planets. The best bit is it also introduces Marauders which add pirates to the game and the Great Khan End Game which is also a bit of variety for the “unfun” last third of the game.
Distant Stars, adds a lot of new events to the “exploration” stage of the game which for me is the best part of Stellaris. Well worth getting if you like that first third of the game.
Megacorps, really didn’t appeal to me. Introducing playing a Corporation focused on making money, but I got bored of Stellaris before really giving it a chance.
Ancient Relics adds archeology to the exploration and also to the main game. Great addition to players that like the science and exploration side of things.
Finally Federations add on to what was already included in the main game. Giving you additional options when creating alliances to face the final end game. Once again, get it on sale if you really want it.
This is just my opinion, if you know what part of the game appeals to you, exploration, research, empire building, combat, end game etc. you can prioritise the DLC you want first.