I would have to go with Kruqed on this one, really.
Speaking from a "Semi-Biased" perspective, I think a Defiler would make a great addition.
Not that any of the priest classes couldn't do a sufficent job. I have a Fury myself and am perfectly capable of keeping a single person alive, and nicely buffed while doing some nice damage.
However, I've recently started a shaman as well, he's only level 16 at the moment, but I've been duoing all the time with a Predator guildmate of mine and we've been tearing it up as a pair.
Shamans have a lot of useful spells. Keep in mind, each class could do a great job, just in different ways.
But if it were me, my "personal" preference would be Shaman -> Defiler.