Depends on play style, Necro/Druid is majorly powerfully, if you don;t mind being limited to kiting, but you have FD, dual snares, healsm, ports, resses and everything.
If you prefer a littl emore traditional and are open to newer thought... I play a shaman / Zerker duo. I like it because zerkers are basically the same HP table as knight classes, good avoidance, and HUGE DPS. Add to the zerks innate abilities a shaman epic 2.0 zerk epic 2.0, and hit a few discs, you can pretty much pull zones like Loping planes 5 at a time.
The old standbys of warrior/shama and monk / sham are as alwayts very powerful, buit in my experience, the zerk just plain outdps's the monk, bu tof course you lose mend and FD.
IMO Zerk Shm is best for a duo because to form into a larger group is simple, add a healer and a tank if the stuff just hits too hard and your set, though a well geared zerk can tank just about anything i have yet found, I regularly duo Fort Mech and Gyrospires fairly effectively, of course not as efficient as a full group. But more XP in duo hehe so even trade off.