I raided with every class except mage in TBC. Started 3 boxing a little before 2.3 hit, then was 5 boxing around Jan after lich king came out. I stopped raiding before lich king came out. I have several 80 teams including mixed teams.
I would have to agree with picking a druid, and my 2nd choice would be an affliction lock. The druid is prime choice just because it can do everything like everyone else has pointed out.
The affliction lock because I enjoy the complexity the dot rotation can bring.