The best DLC for Warhammer 2 is Warhammer 1.
Beyond that it completely depends on what you want to play. The only content gating in the game is in what you can play with. But you can still fight against all the stuff without the DLC. So keeping that in mind I’d say do you like a particular faction? If so, the expansions focused on the factions you enjoy will probably be a good value to you.
Skaven is the only core faction where if you don’t own a DLC you are kind of missing some of their key and faction-defining units from Prophet and Warlock. The Skaven campaign from that DLC has won the several polls I’ve seen as “most fun campaign in Warhammer” by a pretty significant margin. You get nukes. It’s pretty fun.
As for the DLC races, I think Wood Elves and Tomb Kings have really nice campaigns and I enjoy playing them. Vampire Coast and Norsca campaigns are more middling to me but some people love them to death. You can pass on Warriors of Chaos and Beastmen unless you just love the aesthetics so much you can’t live without them.
You get most of the good Wood Elf stuff just from owning Twisted and Twilight which is cheaper than the Wood Elf race pack. And you get some bonus Skaven stuff.
If you have any more specific questions let me know, but thats some general info at least.