If you ask me, brass is bad. French Horns are getting pretty good; if you get the expansion cards like the Roland Orchestral 2 board. Trumpets (and especially supposed brass sections) are usually really bad, especially if trying to do jazz. Guitars are bad. Harmonicas are bad; though chromatic are better than diatonic. Solo strings are bad; cellos being the best efforts but solo violins still sound bad. This is probably because the cello is usually used for more legato passages while a violin would maybe want to dance around a bit more.
I've not yet heard a convincing solo mandolin; though I've heard pretty good mandolin ensembles like in Italian music.