Federation and Nemesis are both good and fix a part of lacking diplomacy system, but aren’t that special. I’d say Nemesis is nice for one game and that’s it while federations are useful always.

From species hands down aquatocs then necroids and then plantoids.

Leviathan is nice, but tiny and I’d rather get the two big dlcs over that. 

Still it depends on you, but bigger dlcs are usually more worth it. And plantoids are simple graphical, other species packs at least add an origin or traits, but plantoids? Nope. If you can’t get everything avoid plantoids.