I tried Civilization 4 but hated it.. even though it's made by the same people (mostly) they took out most of the Automated AI stuff they had in with Alpha Centauri. You can't "Automate" your units anymore, other than explore automatically.
That was exactly my criticism of Civ4. Alpha Centauri was filled with characters that seemed completely different from each other and often quite real. All the AI characters in Civ4 felt like carbon copies of each other and despite being much newer the diplomacy model in general felt far inferior.
Sadly I havenít found a game that matches it depth or character.
You need to play the Fall from Heaven 2 mod. It has very
distinctive AI personalities and civilizations, as well as a more interactive back story (ruled by the Armageddon Counter) than Alpha Centauri had. Religion and alignment are very important to diplomacy. No unit automation beyond workers and building queues, and it does not have the extensive cut scenes and voice acting that fleshed out SMAC. But each civ plays very differently from every other -- much more so than in vanilla CIV.