Which character is the best at support?Follow

It completely depends on how you define support. If you define support as "healing," well, most likely you want to be a priest then, since a priest is predisposed to be a healer. If you define support as "being able to fill any role needed," then you would probably enjoy a druid (which is what I am). As mentioned before though, druids can perform any role (healing, dps, tank) and be very good at any of those roles, but have to switch gear and/or specs to do so.

If you define support as I do, which means "making the other players around you better," then I think it would have to be a shaman. Shamans (ShaMEN?) can add dps and other abilities with totems, can heal, can dps, and can be a temporary tank, all in the same battle. They won't be the best at any of those things, but can help with all of them, making the group better. A priest obviously can't tank, and a feral-specced, feral-geared druid can get only get off a few heals in an emergency. A shaman can do all those things, if needed.

That's my opinion on the matter. I didn't mention paladins, because paladins are bad...

At least until BC rolls out and I can play a blood elf paladin.