Guitar players - today most styles do but Nashville has always had great guitar players. Metal has all the technical speed demons but dont underestimate country pickers. Jazz or fusion has always had the best of musicians from guitar, keys, bass, drums, horns or woodwinds. The progressive metal bands such as Symphony X, Dream Theater, Circus Maximus, to name a few, have the best of the best as well. I also believe the best vocalist exist in progressive metal, progressive rock, country or even commercial air play (pop of what ever genre) bands. Im talking men and woman that actually sing and have powerful, expressive, unique and passionate voices.
So I say for well rounded, deep, intence musicians, progressive bands of what ever style from metal, rock & jazz... plus country for guitar & other strings, even thier bass players and drummers know thier shit but simply just do thier job because they would spoil the music otherwise. and add commercial of whatever style for singers.