There isn't a class I don't like, comparing it back to 3.x. How am I supposed to answer this?
Love Clerics because they can heal as minor actions, or as part of other actions.
Love Fighters because of Combat Challenge/Superiority.
Love Paladins because of Divine Challenge, and full plate + heavy shield at 1st level.
Love Rangers because of Hunter's Quarry.
Love Rogues because of Trick Strike and the like.
Love Warlocks because they are fresh.
Love Warlords because they are fresh.
Love Wizards because they can cast spells at Will.
If someone told me, hey, our party could really use an X, can you play one, my answer would without hesitation be a yes.
I guess Rogue has always been my favorite class to play, and that's not likely to change in this edition, so that'll be my vote, but as far as game mechanics go, I really couldn't pick a favorite.