Vanilla probably had the most "viable" choices to choose your class differently. You'd see hybrid feral/resto druids, and weird mixes like that. There has always been one "cookie cutter" build that serves as the most appropriate for a particular spec though. In vanilla, sometimes you would skip out on the last ability in your tree just because it sucked so much. It wasn't like that as much in TBC/WotLK.
WotLK had some interesting mixes too. Choosing your off-talents made a big difference, so you could get some interesting builds (it was at least interesting PvP-wise). Class diversity was also probably at its peak, since this was before cataclysm homogenized classes by giving all of them CC and interrupts (that wasn't necessarily a bad idea, but it certainly ruined diversity).
So yeah, WotLK or vanilla. I personally prefer WotLK though. You had more abilities, and the skill cap was a lot higher than in BC and vanilla. In vanilla, some classes really didn't have any moves at all (e.g. rets), and things weren't nearly as balanced.