Re: Who will have the biggest career, Stephens, Robson or Keys
I'd say Keys has a higher chance of becoming a top player than Stephens, but she also has a much higher chance of never becoming a relevant player at all. Her game is far too one-dimensional right now, and unfortunately it is much easier to form a solid, average player into a killing machine than a glass canon into a sustainable tennis player. There were so many young players with big game, just missing finesse, a little movement and defense improvement who turned to nothing, searching for that precious "zone" that kept avoiding increasingly.
About Robson, well, I've seen her playing absolutely top quality tennis (much better than anything I've seen from Stephens or Keys), but then again she plays dozens of awful matches without any indication of improvement on any noticeable dimension.
As a conservative, I'd go with Stephens. She already is an established player and doesn't need to change anything fundamentally to become a top player.