So far I think I like the game best at 3 players. You get to build a good engine and the game last long enough if feel like all the increased production buildings have great value. What do others feel?
So far I've only played with 1, 2 and 5 players, and liked every player count. Even with 5 players I felt like I had a cool engine going. But I did play a lot of blue cards, so I had more "bonus" actions and triggered effects than the other players.
But based on my experience, I would think 3-4 players would provide the best balance between having a good amount of cards played per player without it becoming overwhelming to track, and having enough generations for all types of cards to be considered useful if drawn at the right time.