When it comes to personality & charisma, as of right now, Reigns seems like the weakest of the three. That's not to say that he's weak or anything, it's just that his role has been mostly that of the muscle who only seems to speak when he has something to say, or to help drive home a point that one or both of the others have made. It's a role he's done very well in thus far.
All three have been impressive and bring different skills to the table. Ambrose is great on the mic and definitely knows how to use psychology, facial expressions, body language, etc. and weave them all into promos or into his matches. Rollins is the most exciting to watch inside the ring and he's pretty good on the mic himself. He's the more overall athletic "work horse" wrestler of the team. As for Reigns, as I said, he's the bruiser & the muscle of the group; yet his role as the muscle isn't as some one dimensional, low level thug that anybody with good size could pull off.
Reigns has the most impressive "look" of the three because of his very close resemblance to The Rock. Reigns is a member of the Anoa'i family, as is The Rock, and if you were to put long & straight black hair on The Rock along with a goatee, he & Reigns could pass for brothers.