T3 all the way, with some T4 and T2 outliers, which are still more or less powerful than most of their tiermates.

By using T3 classes you don't have to actively cripple yourself to NOT break the game, and you're still very, exceedingly competent at the thing you want to do, while having weaknesses that justify having other people around to cover those. So the game doesn't turn into 4D chess with the DM, and nobody has to actively support a weak link all the time.