[douglas]

Staying within the limits of the stock game, high difficulty high bonus TDM Modpack is the hardest AI you can get for single player. If that's not enough, you could try turning on team mode (all AIs vs you) and not using all of your racial points in designing your empire. Also, don't use tactical combat if you don't have to - the AI is predictable enough that any half-decent human player can defeat much larger AI controlled fleets in tactical combat. This would be suicidal in multiplayer, but don't use training facilities - the AI will never train its ships and fleets except by accident. Finally, try playing in simultaneous movement mode - that's what the AI was designed for, and it's very difficult to run a multiplayer game any other way.

If you're willing to play a mod, the AI Campaign mod available at Spaceempires.net was specifically designed to make the AI as hard as possible, but it uses some very unfair tactics to do so.

Finally, for a real challenge, head on over to PBW to play against some human opponents. BTW, there's a fun game of everybody vs the space monsters still waiting for players (using Adamant Mod) called The Galactic Invasion 2.

Edit: Apparently editing a post seriously screws up the links stuff. I'll try posting the links in a new post.