I play Civ 3, 4, and 5 regularly, and they all appeal to me for different reasons.
Civ 3 was my first Civ game, and I love it mostly for nostalgic reasons. Civ 4 is the best balanced in the series, but I don't particularly care for the graphics in it- I'm just not a fan of that era of 3D graphics. Civ 5 is absolutely gorgeous, and introduces a lot of interesting new elements to the series, but it has had some balance issues, and the mod community probably won't be able to do cool stuff until the SDK comes out.
They're still all Civ though, and they are all definitely worth picking up.