GalCiv 2 is a good way ti go, in that the higher difficulties actually made the AI smarter rather than just adding additional resources to a moron.
It also has the various choices of what you want included in each game.
I don't want an AI that's necessarily more aggressive, just one that will surprise you on occasion with sneakiness rather than just spamming elite stacks. Stuff like cutting you off from your allies, building up it's own coalition and isolating you diplomatically before invading, that kind of thing.
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