it's Rooney or James without a doubt.
There's a few more interesting names that I think have been missed here, as players that never really lived up to their potential possibly. As opposed to someone like Baines and Arteta who of course had talent, but imo got to where they were through sheer dedication to their craft and fully maximising everything they had.
Jack Rodwell - at the time he looked like he could be a complete player at around 17/18 years old. England hadn't seen a natural CB look as comfortable on the ball as him since Ferdinand, and it wouldn't be the case again till John Stones (more on him). Captained all the England age groups. Looked like he could be a future leader for club and country, all the physical and technical tools. Seemed like he was going to emerge as an attacking force after his contributions against City and United at the back end of 09/10. Unfortunately for him his career stalled with injuries and possibly not having a defined role at City. Also was probably too good a squad at the time for him to get enough game time and develop.
Ross Barkley - another player with all the physical and technical attributes and earned comparisons to Zidane, Gazza and Ballack. As raw and brilliant as Rooney was, I think Ross, probably due to his age with better players around him/more advanced coaching methods at this point in English football, showed more frequent and technically superior moments than Rooney did. Looked destined to go to the top, injuries and another bad move and maybe a lack of decision making and intelligence stopped that.
Gerard Deulofeu - quite simply the most jet heeled and mercurial player we had since Kanchelskis. The best player in his generation at La Masia, best player for Spain's age groups when their senior side was conquering the world. And showed incredible potential here. Before his injury in 13/14 I think we would've had a nice big points gap to defend a champions league place on, he was ripping teams to shreads. In 15/16 from around October to new year he was the stand out player in the premier league. he was having a better season than Mahrez and Vardy, but wasn't getting the team results they were. Unfortunately for us Martinez replaced him with Lennon after new year and Koeman hated him.
Manuel Fernandes - had it all, the touch, the technique, passing range, could shoot form any angle. Again, just mising that something to set him aaprt from the rest mentally. The Valencia move another slidign doors moment.
Andy Van Der Meyde - even when an overweight addict going through a terrible depression and trauma with his daughter in hospital, he was the best technical player in a side including Baines, Pienaar and Arteta. Whenever Moyes did give him a minute his quality with the ball always shone through. Over than Geri, the best crosser of a ball we've had in the premier league era imo
James Vaughan - but for injuries i think he could've been an incredible striker and target man. he wasn't a natural finisher but he made lots of opportunities for himself and his speed and ability in the air was quite unique at the time. I remember him having a spell towards end of 07 season in which he was used as the target man and was dominating the likes of Terry, Ferdinand, Mellberg, Zat Knight and Allardyce Bolton groks in the air. Haven't seen anything like it in the air until DCL, and he's a good few inches taller than Vaughan and not as explosive over the ground.
John Stones - hadn't seen anything quite like him at the back, cool, classy, and totally unfazed playing cente half as a teenager. his development at city speaks for itself
these are the more interesting players for this kind of thread. Not the likes of Arteta and Baines who always come up in best player discussions
These are the players most inte