Biggest never-seens for me, apart from the obvious like Zeppelin, Waylon Jennings and Hendrix, is probably... I don't know... Queen, Anthrax, Bruce Dickinson and Scorpions.
I've seen Maiden (2), Priest (2), Crue (2), GN'R, Purple (4), Ozzy, H&H (3), Aerosmith (2), AC/DC, Alice Cooper (4), Skynyrd, ZZ Top, Metallica (3), Slayer, Megadeth (3), KISS, Dream Theater (4), Queensryche (3), Slipknot (2), Bryan Adams (2 - that's right, fuck you), Bon Jovi, Alice In Chains (2), The Cult.
That's in terms of stature, though. Biggest never-seens for me personally would be: Children of Bodom, Corrosion of Conformity, Days of The New, Dark Angel, Kreator, Flotsam & Jetsam, Forsaken, Godsmack, Green Carnation, The Hoax, King's X, Richie Kotzen (rectifying in October), Yngwie Malmsteen, Molly Hatchet, Gary Moore, My Dying Bride, Axel Rudi Pell, Brainstorm (rectifying in September), Symphorce, The Quill, Rage Against The Machine, Savatage, Stevie Salas, MSG, Sixx:A.M., Spiritual Beggars, Ray Wilson/Stiltskin, Swallow The Sun (December), Pentagram, Brian Vander Ark, Venom, Witchery, Y&T, Rob Zombie.
And now won't ever see: Rainbow, Pantera, Reverend Bizarre, Slash's Snakepit (although maybe some stuff on Slash's solo tour?), Talisman, White Zombie.