Bizarrely enough, I saw Mastodon supporting High on Fire, around 2000, and Mastodon just blew High on Fire off the stage.
Saw Metallica on the Justice for All tour with Danzig as support. Metallica were boring, Danzig were superb.
Slayer's South of Heaven tour, went for Nuclear Assault, left the Slayer set after the first 3 songs.
The Wonder Stuff 1987/88 ish. To be fair, TWS were really, really good, but the support band were just superb... Zodiac Mindwarp & The Love Reaction.