I tested engineer, runie, ironbreaker, chosen, squiggie, shaman, sigmarite, witch hunter so far.
They all work in PvP and RvR, sometimes better, sometimes not as good - depending on the group and situation.
You gain renoun points for protecting flag bearers too, even if you do only small amounts damage or healing, or - i.e. in the Troll Crossing scenario. This is great for engineers with their single slow and AoE root, or ironbreakers on full protection outfit. Squig Herders can win the scenario for their side without even coming close to the pacifier with their long range snare.