I'd say either Forsaken or Draenei. I don't think any race has had it worse than those two.
Draenei - betrayed by their leaders (except Velen) and exiled from their home planet. The few survivors from that are constantly hunted down by the legion to the point where they can't stay in one place for long. Hunted down and mass slaughtered to near extinction by demon-crazed orcs, who suddenly started attacking them with no warning after the two races had lived together in peace for years. It seems like every time they manage to settle down and live in peace somewhere, bad shit happens again and forces them out. Also there is a long road in Hellfire Peninsula that's paved with their bones.
Forsaken - dead people who were reanimated to serve the Lich King (slavery) and fight for him to kill their family/friends. They got their free will back, but they are now eternal walking corpses with vague memories of their past lives who can't be accepted by anyone else. Aside from a few exceptions, their family/friends from their old lives want nothing to do with them now, and even their own allies (the horde races) don't particularly trust or like them. They were forced into what they are and never asked for it.
Vote goes to Draenei I think. As bad as they've had it, they're still nice/friendly to everyone else and put on a good face. Forsaken are just hateful and bitter.