I like to solo difficult stuff using a full hit table and high BV. Some fights shine here, and some (namely any with substantial magic damage) don't. In particular, I used to kill Arlokk every 3 days (until the loot dried up and the path to exalted seemed too long to bother), and Garr every week (which I'll continue to do until I get my bindings, and perhaps the recipes I don't have yet).

Oh yeah, and back in the old days (pre-3.0) I could take on the Infectious Skitterers in Naxx...

Timetheos