Civ4 is probably the best game, but Civ2 is a bit more simplistic (although some things in that game are even more complicated. There's a lot of micromanagement that's been simplified over time). Basically, Civ2's problem is that the AI is terrible (which can help overcome the learning curve problem because you can make mistakes without problems). Civ Revolution's not a bad choice (I would assume, I haven't played it). It's basically Civ4 simplified.
Civ3 with the Conquests expansion is not a bad game. It's closer to Civ2 with updated graphics, although there are plenty of changes as well. Honestly, I think Civ4 is the most diverse game in the series, but they're all great.