Civ III was the only installment that I did not care for that much. Civ V was a bit weak before the first expansion, but that was so long ago, and I have played it so much since the expansions were released, that the pre-expansion game is but a distant memory. The AI is less than impressive, but if you let it cheat like crazy (set it on higher difficulties), it can be challenging. I typically play on Emperor these days, and haven't lost a game in quite some time. Some of the scenarios can be quite challenging as well, if you play the right Civilization. It took me forever to beat Out of Africa, even on King difficulty, until I played as Egypt.
Anyway, I'm looking forward to Civ VI coming out in October, but first I still have several civilizations that I have not yet won with in Civ V.