As far as base classes are concerned, from what I've seen I'm most excited by Monk and Fighter. Their subclasses really demonstrate, more than any others I feel, just how varied two members of the same class can be.
Favorite subclass, however, gets split between Battlemaster and Wild Mage. I love tactical fighters (most of my sadly short time with 4e was spent as a Taclord), but nothing stoked my imagination as a child as much as the 2e Wild Mage. It's been much too long since we've had a proper Wild Surge table.