If Aguero is not world class then neither is Tevez and yes Aguero is better than Tevez and will be much because he's still 5 year younger so i don't how you come to the conclusion that he'll never be as good because he's already better.
Aguero is not better than Tevez. Tevez is a much more dangerous player and if I really needed a goal in a match, I'd no doubt choose Tevez over Aguero. Aguero may surpass Tevez in a year or two though. But at the moment, Tevez>Aguero
And to Melo, I usually think it's the top 5 or so for each position with some positions having more, maybe ~10 (CM, CB, AM, ST and GK)
World class fowards at the moment in my opinion are:
Messi
Ronaldo
Villa
Eto'o
Tevez
Rooney
Ibrahimovic
Torres (in form)
Aguero is in the group of players like Cavani, Suarez, Van Persie, Drogba, Lisandro, Higuain, Benzema and Forlan