Back when the group first formed and for the most part, until their final months, they were very similar. But Dean Ambrose had the most charisma and the most unique style. Rollins did flippy shit and Reigns did the power moves, but Dean did both. Reigns was more menacing, but Ambrose was the man.
After the split, Seth Rollins came into his own more. He became a better character, and really gained more wrestling experience and became a work horse. Dean Ambrose, was the most underrated after the split. He seemed to be shafted down, for more work, but Reigns was the one that needed the most development.
I think Seth Rollins is the best because he's the best of both worlds. It seemed like he took the style of both Reigns and Ambrose and compounded it into his own unique style added in with his own charisma and work-horse status in the company. Dean Ambrose still has my respect, and I love his title reign. He's been running hard with the title for months and is drawing good houses. Much respect for Titty Master.