I wasted so much time on Civ 2. Amazing game. I bought Civ 3 last year, and while it's good, it seems like the same game with a (very nice) face lift. Sure, there are some gameplay improvements and additions in some areas, but there are also subtractions in others. As a result I didn't spend nearly as much time on it as it felt like I'd already played it thousands of times.
Civ 4 really, REALLY, needs a complete overhaul when it comes to diplomacy and trade agreements. I mean seriously, these areas are so limited/flawed/broken, and they should be two of the focuses for the series. Also, Civ 4 looks like shit graphically. Not only is the 3d technically shite, but the style is fucked.