I haven't played it; never cared for space strategy; but Galactic Civilization is said to have a very good AI. Of the games I have played, I would have to say that SM:G would be the best AI, but I always suspected it of cheating somewhat; even on normal difficulty. Lords of the Realm 2, could at times, surprise me. Shogun was good after the patches. We can only hope that RTW will follow the same path.