Enchanter.
- Haste (by far the best melee buff).
- Can slow nearly as well as a Shaman.
- Tash.
- Ultimate CC - mez/stun/root.
- Can pull with the above and Pacify if needed.
- Charm - best DPS in the game.
- Rune.
At 60 it gets a bit closer because Shaman has pretty good healing power with Torpor but when it comes to 1 class making a huge difference in a group nothing comes close to an Enchanter.
Things are a lot different if we're talking about raiding, though.