Carson is a successful brain surgeon. A self-made black man from Detroit. He's like a unicorn. His inexperience notwithstanding, I would vote for him in a heart beat if he received the nomination. He is gaining in popularity because he has personality and speaks like a regular person, not some shitbag political hack pandering for votes.
Trump is the diametric opposite of Carson in every single way-- but they are both at the top of the heap because they speak fearlessly and are unapologetic about their beliefs.
IMO Fiorina is the best candidate of the bunch. She is polished, intelligent, successful, not bad on the eyes and knows exactly how to articulate her positions so that they resonate with the average voter. Shes not a politician per say, but she does know how to play the game.
Those three aside, don't be surprised if Cruz is the last man standing. A presidential debate between Cruz and Hillary would be like watching a snuff film. The lib moderators would have to cut to commercial break every 2 minutes and shut Cruz's mike off at random to save Hillary from being dismembered on national television.
Those are my 4 picks in no particular order.