I will have to second the Cosmic Encounter choice. I've never played a more fun board game to date. An interesting note: this game was actually one of the principal inspirations for the card game that eventually became "Magic: The Gathering."
But, don't take my word for it, try out the online copy of the game and decide for yourself. The free version is a little sparse, but if you decide you like it, get the real deal. ;)