Quad shamans with someone else on the healer are amongst the top for 5v5 Arena.
Just about any composition works for battlegrounds, but shammies are relatively strong there too.

Four shammies and a prot paladin is probably the easiest combination to do heroics with.
Again shammies are strong in PvE too.

They wear mail armor and equip shields.
They can heal and ressurrect and self rez.
They have a multitude of totem buffs.
They have amazing burst.

Lots of nice toys...
Earthbind to snare/root.
Windshock off of the GCD for extra spell interrupts.
Fire Nova to stun.
Fire Elementals for AoE (not in arena).
Lightning Shield vs cleave/aoe.
Thunderstorm as a pushback or push over the cliff.
Instant ghost wolf.
Grounding vs casters.
Staggered Tremor totems vs the Fear Bomb.
Etc.

They're not the only team with success, but they're up there.