It all depends on your goals.

If you'll primarily be in battlegrounds, just about anything will work and be strong boxed. Fear bombs are your primary threat in battlegrounds. Fear Ward, Trinkets, Tremor Totems, Berserker Rage, there are counters to fear. You can also destroy anyone who you notice running towards you. They will hit fear on you now and then, nothing you can do about it, but its not the end of the world. With "Click to Move" and "Interact With Target", you can have all your toons running back to a central point with ease, so the fear is inconvenient but not the end of a fight.

For BG's, 4x Druids (Balance) is very strong. You could run the DK in a blood spec for decent damage and heals. When necessary the druids can 4x HoT's. They have good damage, Wrath, Moonfire, Insect Swarm, Starfire, Starfall, Roots on round robin, Force of Nature, Hurricane, Cyclone etc. The DK wouldn't want to be tank spec, since you cannot force someone to attack a tank. Four pallies with Click to Move/Interact would be a force as well.

Arena requires a lot more coordination and your composition becomes quite important. Boxed, pretty much the only teams that have been very successful seem to be 4x Elemental Shaman, plus 1x Healer (played by someone else). There are people who 5x Shaman themselves, but they don't seem to be as successful, although I remember reading of a Gladiator team a couple seasons back. Other compositions I'm not sure of, at the high end, at least when boxed.