I don't know of a single class that is overpowered, but the Shaman-Warlock duo seems to be quite formidable when put in a three's team, as they have amazing synergy and seem to work with any sort of team composition, whether it be another melee DPS, ranged DPS, or healer.
Frost Mage. Always Frost Mage.
"MMOC forums let me keep my job again. Whew." -Greg "Ghostcrawler" Street
I don't think there's one single overpowered spec, there's just a lot of really annoying ones. Like hunters. Argue all you want they're not op in 3v3 but those random whacky 300k chimera + aimed shot combos scream 'skill' -- as much as warlocks with ten million fears or warriors with ten million stuns.
I'd say top 3 is warlock, shaman and druid. Purely because of the annoyance factor.