Any combination you want to work with, will work.
Some will take more effort then others, but given the time and macro/keybinds anything can work.

If you're playing just yourself, you need to be self reliant as much as possible.
With three toons, all dps is viable in arena, if you plan to include two other toons (including at least 1 healer probably), both played by other players.

In battlegrounds, three dps is viable too, but the healer will likely get you a larger boost.
An affliction warlock gets Haunt and Siphon Life for passive heals, just by DPSing.
I know there are blood talents for a DK to heal quite a bit.
Not sure on Ret getting heals just by dpsing, but they do have healing capability.
If you can down things fast enough, then you can recuperate.

I personally love Discipline priests, so would lean that way.
Discipline can dps ok, but that's not their strength.
You get mana burn, fear ward, pain suppression, penance, mass dispel, etc.
With a small investment into shadow, you can add mind flay quite easily.

The druid is tempting, as you will have Rejuvenation and Lifebloom which can be cast on the move.
Regrowth and possibly Wild Growth as well, if you go deeply into Resto.
Pre Wrath, my favorite spec for a druid was Dreamstate.
Deep enough in Balance for moonkin form and dreamstate.
And then pick up Swiftmend in the resto tree.
Then a few points left over to choose between Faerie Fire, Improved Moonkin (Spirit to damage) or slightly better heals.
A dreamstate druid loses Wild Growth which is a very powerful AoE heal, but gains a not insignificant dps capacity, easily far ahead of a Disc Priest.

I'd still go Disc personally, but its a tough choice between the two.