Actually, I'd say Metallica is the only truly world-wide Metal phenomenon these days. Yes, Maiden has been successful in headlining major festivals every summer, but they are much bigger in Europe than in N. America, where they can headline arenas and amphitheaters, but not stadiums.
I'd put them like this:
Metallica: Stadiums
Maiden/Ozzy: Arenas & Amphitheaters
Slayer/Megadeth/Judas Priest: Smaller arenas and package tours
Many of the other classic metal bands will headline theaters and clubs (Testament, Anthrax, etc).
You can throw in a lot of the current popular metal bands in the category of arenas/amphitheaters, often with package tours. Bands like these include Disturbed, 5 Finger Death Punch, Korn, Avenged Sevenfold, etc.