There's Space Empire IV. It's simplicity and mind bogglingly huge tech tree is addicting.
The only bad thing about it is that some techs (awesome techs, might I add) are rarely used, since by the time you get them and the resources needed to use them, you already have no credible opposition in the game.
Space Empires IV was a lot of wasted potential in my eyes.
The AI is outright stupid... braindead even, offering little challenge at the start and practically no challenge by midgame, which turns it all into a mindnumbingly boring and tedious cleanup operation one system at a time.
Multiplayer... is worse. We actually tried to get it to work on a lan session once but the netcode and multiplayer mechanics really only resulted in excruciating painfullness and annoyance lol.
GalCiv2 is still the very best 4X game to play vs. AI in my opinion. No other games AI even comes close.
Multiplayer Sins has a draw simply due to how much more suited real time games are to multiplayer compared to turn based solution ... and i'll definitely still play classic MOO2 at a lan even today rather than torturing myself with Space Empires heh.