Totally late to this thread, but I'm a huge Civ fan and need to throw my two cents in
I love Civ IV BTS...by far the best. CivIII was great, and Civ V is good, but IV BTS is certainly the most balanced and has the most features (religion, corporations, random events, etc). For me they changed too much of the core mechanics of Civ in Civ V which made the gameplay seriously lacking IMO. Not to say the game isn't good, but I don't see myself playing it for hours on end like I did with III and IV. Anyway, looks like you've already purchased V, but if you don't have it you should seriously consider picking up Beyond the Sword...epic stuff!