Leighton Baines is world class in my opinion, because world class to me means you are one of the very best in the world in your role. If there are any better LBs than Leighton, they're not better by much.
Coleman has the potential to be world class, but is already excellent and can only improve.
Ross is on the fence for me. World class potential, but does he have the ethic and discipline? I think he's better in that regard than Rooney at his age, to be fair, though some might argue Rooney isn't/never was world class.
Lukaku again, world class potential, but time will tell. However any striker with a 1/2 ratio in the Prem is a 'top' striker. If he doesn't become world class he will have wasted his potential, and from what I know about Rom, I think he's got the right attitude to do everything in his power to achieve that potential.
McCarthy, if he balances his game out a bit more, can get there too.
John Stones will be world class. I have no doubt. The only way he won't be is through injuries. He has maturity, ability and an outstanding sense of professionalism.
Ultimately though it's a subjective and arbitrary distinction. Ultimately if the above players fulfill their potential, they will be stonkingly good players. So let's hope they do.