MOO2.  Play a race of your choosing (or making), and conquer the galaxy by vote, military might, or by being a hero (defeating the evil enemy that hates everyone). A great tech tree and good flexibility in ship design and  colony building. The game has great strengths in its interface(s) and the  many choices you can make during play. It does devolve into a bit of a tech  fest, and there is little emphasis on diplomacy, but an all around good  builder game.