Bards are the strongest for multiple reasons. High (very high) damage, River of Blood is stupidly high damage, all our personal buffs are great and amazing damage, we couple white hits with out abilites which casters do no.
Basically if bards don't sing a song they are untouchable high DPS, and when they do sing everyone can catch up but they are still not as awesome as a bard because they aren't singing a lovely song thats making everyone else perform better.