Since they tied for most votes in the "Favorite Bassist" poll, I'll review both Cliff Burton and Flea's contributions to the art of bass guitar.
Cliff Burton: Cliff is a true tragic hero for bass guitar in heavy metal. Having lost his life in a bus accident while on tour at such a young age, it is hard to speculate how much more he could have brought to the world of heavy metal with his bass guitar. Even in his short career with Metallica, he contributed significantly as a song-writer and made his unique bass-playing style prominent on many tracks. To hear some of the best work of this master of bass listen to:
-Anesthesia (Pulling Teeth) This instrumental from Metallica's first album, recorded in one take, showcases Burton at his best!
-For Whom the Bell Tolls
Flea: Flea has made the tonality and technical playing of his bass guitar a signature trademark of the music of the Red Hot Chili Peppers. From funky, slapping bass techniques, to melodic strings of notes, Flea can play bass as good as the rest. For some Chili Peppers songs that highlight Flea's bass try:
-Soul to Squeeze