31st St. certainly for the headbanger in you. No RnR barhopping trip to Pgh would be complete without Gooskis - best jukebox in Western PA, IMO, and raunchy dive bar bands in the back. For trendy/good drinks, The Firehouse.
Without anymore info as to where you will be I'd recommend going to the South Side. There are about 120 bars within walking distance. I'm not kidding. A great trendy "5star dive bar" with a great variety of music is Howlers in Bloomfield.