My friend Charlie has one of the best farming/powerleveling characters in City of Heroes. I say that with no hyperbole; it may be mathematically impossible to beat his build. He has the standard Fire/Kinetics/Fire Controller farmer, and he has every purple ultra-rare enhancement that he needs. To translate for players of other games, he has the best possible equipment, and there is an endlessly repeatable group quest that his class can solo with ease.
His girlfriend Nicole complements this with a character that could otherwise be suboptimal. Archery Blasters are not that popular, but they do come with the perfect nuke for farming: Rain of Arrows. I do not know her slotting (she must have her own set of purples by now), but between her powers and his buffs, her recharge on Rain of Arrows is twenty seconds at the most. Because of his Fulcrum Shift, she can live at the damage cap. While he is crushing one group, she takes out the next one in two attacks. Re-Fulcrum Shift off his next group and continue.
My latest character (Sonic/Sonic Defender) makes this even faster. When you are at the damage cap, enemy resistance debuffs are the only way to increase your damage further. Negative resistance becomes a damage multiplier. Disruption Field on the Controller, Howl at the enemies, and they are at -50% resistance. Add another nuke, plus some shields to deal with return fire.
Farming can be exceedingly dull and repetitive, but it hardly has time to when it goes that fast.