Maiden, hands down.
If you had asked me this question around 1995, I'd probably have said Metallica. After all, Bruce had just left Maiden. But after the abduction of the real Metallica and subsequent replacement by their evil short-haired doppelgangers in '96, it's not even worth debating at this point...
Metallica has gotten so damn lazy with their chops and songwriting, became the corporate whores that they always fought against, and simply lost touch with their original fanbase.
Meanwhile, Maiden has improved and not regressed over the past half-decade or so. They got Bruce back into the fold (even though Blaze wasn't a bad replacement) and never stopped proudly waving the flag of METAL! There was none of this revisionist, "we're a rock band, not a metal band" crap. And they can play circles around Metallica at this point. Kirk and Lars' playing, in particular, have both deteriorated so much that it's tragic. Meanwhile, Maiden can still rip through their whole catalog easily. Bruce may not hit as many air-raid shrieks anymore, but other than that, where's the dropoff?