SEIV Gold has a failrly rubbish AI - OK for learning the game but not really challenging once you're up to speed. However there are a couple of mods which have made the AI opponents pretty tough. And they don't rely on cheating.

GalCiv's key feature is its AI. It's the only game I've played where, when you increase the opponent difficulty level, your opponents actually get cleverer and sneakier. On the downside, it lacks the complexity or flavour of either DomII or SEIV.