Cosmic Encounter [1] is my all time favorite. I think it works best with 5 or 6 players, which allows for lots of interactions and wacky card play. Some of the alien powers are better than others, which might seem unfair. But the fun is in assessing each situation and making the appropriate alliances, etc. to "balance" the game - hopefully to your own advantage, of course!

[1] Specifically the Mayfair edition from the 1990s, although I understand the current version is pretty similar.