I'd personally say Prot Pally, but I haven't played too much Prot at 20. Vengeance DH falls off because of not a strong self heal and lack of mitigation imo. At 10, Prot, Guardian, and BM Monk are by far, a few of the best bets to solo bosses. Monk is also great because of Touch of Death, but their DPS is also considerably lower. I had trouble with the Hunter/Lock/Mage, the pets just couldn't hold aggro on the first boss of EoA, and Mages aren't tanky enough to burst a boss down