contemporary r&b - beyonce
hip-hop soul (not the same as soul by itself) - mary j blige
hip-hop/rap - grandmaster flash + the furious five, eminem (he's pretty good

) Dr Dre, Jay-Z. In hip-hop the list is massive.
pop - MJ and Madonna
soul - james brown and aretha franklin
dance - hard one, anyone can claim the title...madonna/rihanna for that one. and the black eyed peas (people hate them) but they have created some serious ultimate party tunes.
hard to say...beyonce is stunning in regards to contem. r&b, performs with the ferocity of MJ and james brown and has them killer vocals. for an r&b artist now, she's the one to sit and take notes from. in the future, lady gaga will try and give madonna a run for her money - i like her but if she wasn't using a lot of madonna's ideas, then she'd be amazing. no one can be entirely original, but you know what I mean. Nicki minaj will be a force to be reckoned with in the future, a female rap artist with pop, hip-hop & r&b appeal. they don't come around often...if at all.
not a fan of altern., classical or rock so I cant say on those.