Sylar

He is the best boss killer there is, dealing massive damage against bosses.
*His basic attack goes to around ~300% against bosses, not to mention the skill he casts every 6 attacks.
*Equip him with the correct Root gear, and he will deal brutal amounts of damage.
*He is a favourite against Domain Bosses, Alliance Bosses, Cube Bosses, and Event Bosses. For those of you who don't know, that is more than half the regular content this game offers.