I think Darkdrift Knight and Night Aspect are the most satisfying. Middle-sized solo bosses have always been my favourites. But I'd say all fights are pretty cool, even gank fights are designed well.
I'm playing mostly in a group. With randoms, but planning to do with friends. For me it's more fun then solo and adds to the experience. But learning the map, characters and mechanics is better solo. And guide videos are invaluable here, since the learning curve is so much steeper