After hip hop, these 3 are my fav's: Jazz (mostly instrumental jazz, esp. Avant garde, classic and bebop eg. John Coltrane, Miles Davis, Ornette Coleman, Sun Ra, Dizzy Gillespie, Charlie Parker, Thelonious Monk, etc.)
Rock (mostly classic rock, esp. artists such as Carlos Santana, Jimi Hendrix, Credence Clearwater Revival, the artists of the British invasion like the Beatles and Becky, Ricky, Mick, Tick and Tosh or w/ever the fuck they are called, etc.)
Classical (not necessarily referring to the actual classic period during the 1800's, more like everything that you would see at a concert hall from the middle ages to now. Eg. Beethoven, Igor Stravinski, Gavril Popov, Sebastian Bach, hell, even some ancient Gregorian Chant is cool at times. For some somewhat modern singers, Luciano Pavarotti (recently passed) and Placido Domingo are quite amazing)
Hon. Mention:
Alternative (Herb Alpert)
Pop (only a few new artists. Michael Jackson, Prince, Leona Lewis, Christina Aguilera, etc.)