A lot of people are recommending Ideology specifically here, and while that would also be my own recommendation, I feel like I need to throw in a caveat.

Ideology is an entirely hit-or-miss DLC, because the entire point of most of it is giving your colony various limitations for roleplaying purposes. If you don't care about roleplaying then it's probably one of the last DLC you'd want.

Though it does come with the advantage of being the cheapest DLC.