Mostly the English/British players. Theo Walcott is overrated by many imo, also Jack Wilshere despite being a very good player, and one with huge potential, his potential is quite overstated in the Arsenal forums i go to. Then you have the typical overrating of new signings, with some posters going as far as to claim that Gervinho is better than Hazard, using stats and stuff like that.
But the player that is most overrated imo, is Aaron Ramsey. I don't know, i personally don't rate him that highly and believe he's a limited player, most posters on Arsenal forums believe he has huge potential.