I've seen all the big Metal bands over the years. Usually more than once.
Most proud of:
Celtic Frost - circa 1987
Black Sabbath w/Dio - 1992
Slayer - 4 months after Reign in Blood came out 1987
Soundgarden and Pearl Jam - 1992
Pagan's Mind last year at ProgPower X
just off the top of my head.
Feb 27: Voivod