Leader wins it for me. I absolutely hated playing healers in 3.x, it seemed like the dullest thing. I found Wizards and Rogues to be much more interesting and exciting. But with the changes made, Leader has become the most interesting class for me.
Strikers and Controllers come next. I can't pick which one I like better than the other. Whenever I play either, I tend to lean toward a secondary leader or controller role. I like team play even as a striker.
Defenders are likely my least favorite to play. My problem with defenders is, I see fighter, and my eyes see nothing else. Every time I look at a defender class, all I can think of is how a fighter can do it better. That one dimensionality has me currently turned off from the defender role. However I always want one or two on my team, so I'm grateful for those who like to play defenders.