Johnson's defensive vulnerability is overplayed. Few goals come down Liverpool's right - most this season have come from our left-back position [edit: in fairness, those have dried up since Konchesky got bombed out of the squad] and have done for a couple of seasons now (from open play).
Currently, I'd say there's not much to choose between Smalling, Wilson (Scottish young player of the year last season) and Ayala (who's impressed both at Liverpool and more recently at Hull). Kelly's a bit further ahead, but then so's Johnny Evans.
Smalling's probably going to be a better player than Wilson and Ayala will develop into if I'm honest, but at the moment he's still got to develop into that player. That's purely because he's got the technical ability and the physical attributes in combination. Wilson's got a lot of filling out to do, and Ayala will never have the pace of Smalling. But then Smalling's positional sense is nowhere near good enough yet - that'll improve with yet more experience you'd hope.