I was going to list them (eg. Stones, Beatles, U2, Oasis, Arctic Monkeys, that horse-faced crackhead with emphysema), but I'd just say that the most overrated bands are usually the ones that 'hip' journalists rave about. They're obviously being paid to plug a label's new band, or they like to play 'trendy' and plug some godawful no-hopers, which the others then jump upon as the new music gods.
I lost count of the bands I listened to who were being touted as the new wunderkinds, and who were absolutely dire / derivative / overblown tosh.
Now I mainly listen to unsigned and independent bands, as most of them are putting out music that puts the mainstream acts to shame.