For me it's probably Sterling. Although Stones is obviously another candidate.
I never felt confident with Sterling putting chances away, didn't matter how easy it was. Always got the feeling he was going to cock it up somehow, despite all his ability. Now he just seems much more settled and ruthless and it's made him twice the player for me. Even when on the odd occasion he messes something up, he just keeps going at it rather than letting it affect him. Still got a way to go, like a number of our players but the rapid development is fantastic to witness.
Another one is Fernandinho, i thought he really struggled at times last season (although getting shunted about different positions obviously didn't help). He wasn't making the passing angles, was getting caught in wrong positions, playing the wrong passes etc, but he looks like he's really got to grips with what's needed of him now and has been immense so far this season.