Yaya is not the greatest ever by any means but as others have said, he would very possibly make the City all-time XI. I do think that he would have stood out in previous generations.
No-one else today has his combination of size, pace, strength coupled with incredible touch and footballing nous. On his day, he is simply unplayable. You can take Silva out of a game - good and all as he is, but not Yaya. Despite the world-class players that we have: Kompany, Silva etc., only Yaya (and Aguero to a slightly lesser extent) have the "fear factor" to other clubs.
What manure, Chelski and especially Arsenal wouldn't do for a player like him. The nearest thing to Vieira in the game today. It is no coincidence that Arse have won nothing since he left.
So yeah, he's pretty damn good. Sometimes frustrating and lazy, but I always breathe a sigh of relief when his name is on the team-sheet.