For a tank (raid):
Run with Battle Shout if your group is good on HP, Commanding if you need the boost (boss fights especially I will put this one up).
Flask: Flask of Fortification or Flask of Chromatic Wonder (if you need the resist badly)
Food: Spicy Crawdad or Fisherman's Feast
Potion: Ironshield Potion (or Great Stoneshield Potion if you're on the cheap)
Hunter: Misdirect is phenomenal if used correctly. Yes, I know this doesn't really count, but I <3 MD.
Shaman: Grace of Air (mitigation) or Windfury (threat). Be aware that Windfury will cause additional parry procs by bosses that can parry. Whether or not it's worth it would depend on the exact boss and your healer's ability to keep you up if the boss parries. E.g., I wouldn't recommend Windfury on Prince Phase 2 (GoA for the dodge and armor would help keep you up).
Paladin: Blessing of Kings and/or Light, depending on the situation - usually Kings, Devotion Aura in most cases
Warlock: Blood Pact and Fire Shield can be useful if they don't sacrifice their pet
Druid: Gift of the Wild, Improved Leader of the Pack (Feral)
Priest: Prayer of Fortification, often Prayer of Shadow Resistance