I've never updated past 1.6.4, so that is pretty self-explanatory; part of it is the world generation, especially the underground. 1.5.2 (I started on 1.5.1) is close though; the only feature from 1.6 that I've really used is coal blocks (once 1.6 came out I started mining all coal, nearly tripling all ores mined) and there is no regional difficulty (it behaved as if it were always at 100%; for somebody who always moves around it is a negative, and I've modified/replaced it) or server lag due to zombies pathfinding to unreachable targets (the only issue that I've had with 1.6, which I fixed long ago; the changes to zombies make it more fun though).
I liked Beta 1.4 most (probably not because of the update itself, but my experience on it). I first started playing around that update and had no idea what was going on. It was fun to discover features on my own for the first time. Now that I know how most of the game works, it's pretty easy to predict what's going to happen, so it's not as interesting.