I think that the single-target damage king is a Demon/Storm MM. If you can stay alive and keep everyone alive.
Two powersets that do high damage, and also a good amount of -Resist in each one to multiply the damage done by both sets.
My calculations had one at over 500 effective DPS (pre-Incarnate), without really trying for a super build. Granted, Storm is Endurance-intensive, and I'm not sure that it could maintain a full activity set of bringing up Storm powers and also blasting for the -Resist.