I would choose a healer. I will be dual boxing my Priest and DK but think a Pally and DK would be even funner. I just cannot bring myself to play an elf. Shammy totems and heals are also good but may require a bit more management. Druid would probably be the easiest to manage as you Moonfire/Stack HoTs/wait for next mob really with priest being a close second. Just my opinion though, I would love to do DK/Pally if I could make an UD Pally some day...