Excluding Arc and MGT, anything BC. They're pure money trips. The first time I solo'd Mech (my first attempt at a 70 instance solo), I seriously debated grabbing enchanting.
Onyxia was a little tough for me, I was doing well until the whelps came out. I enlisted the help of a hunter friend and we managed to two man her.
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"Do you know what "nemesis" means? A righteous infliction of retribution manifested by an appropriate agent. Personified in this case by an 'orrible c*** : me."