Out of all the classes I tried, I liked the Warrior Priest the best. Reasonable damage, great survivability and generally ignored in PvP due to being hard to take down. Nice to have the choice of being on the front line, DPS'ing while healing/buffing at the same time, or hanging back going for all-out healing.
Pure (Rune Priest, Archmage, Zealot) healers are hard work in T1, your target No1 in PvP and survivability is poor until you get more abilites in T2.
Tanks are always in demand, but I found solo'ing to be hard work with them, especially past T1. DPS classes are always are good choices, but they're very popular for obvious reasons. Squig Herder and Engineer stood out for me.
Can't comment on servers - one of the busiest was Karak Azgal, but I've let my sub lapse until 1.3 comes out, then I'll come back to it. At the moment, keep seiges are pretty dull...