Well I'm not a huge video gamer, but I've been playing the Civ series all my life, and I'd disagree with CIIv being the best of the series. It's either CIV or CiV, depending on your thoughs on 1UPT (1 Unit Per Tile).
If you're going for "historical impact", obviously the first one is the best, I mean yeah CIIv was probably a bigger upgrade than any other one, but still. But today ? I own all of these, and the only reason I would ever play CIIv would be the nostalgia (the music especially).