Good vs Evil? Well an entire party of Paladin or Anti Paladins would be over kill. Alignment Channel, gunslingers if allowed are always high DPS. Magic Circles/Protection from alignment are always helpful. Just some ideas, I'll be back with more if I can think of any.
Wizard, Cleric, Master Summoner, Synthesist-Summoner/Paladin/Barb
A lot depends on what you have to work with. For example, if you must use a Core race (all humanoids) a caster with Charm Person can make an opponent "switch teams" giving your side a big boost. But if non-humanoid races are in play (ex. tiefling) it become less useful.
A Witch with the Slumber hex can target an opponent to remove them from combat.
A Ranger's favored enemy could be good if you know the racial makeup of the other party.
PrinceDogWater's suggestion of alignment based attacks is good, since you know the other party will be of a unified (and opposed to your) alignment.
Zen Archer would be one of the best PVP class, supreme movement speed and archer together makes it almost impossible to reach if the area is big enough.
Summoner would be good as well because you can summon and cast spells. Master summoner would be better at taking down casters and DPRs of another team with creeps that you summon.
Wizard or/and Cleric would be nice. But that means you will need a DPH tank to defend them from the like of rouge/ranger/creeps from summon with Attack of Opportunities.
That's all I have in mind for now.
Grappling. Get someone with this and potentially make one of the enemies irrelevant as they can't get out of it.