Definitely Maiden. Even though I think their first two records were the best, their work with Bruce Dickinson and even some of their recent stuff is all great, and they have always played with such finesse. Whereas Metallica is a smash-your-face-in kinda metal band, Iron Maiden has always been smooth, subtle, alive with a kind of electric energy and acidic sound but encompassed by solid technical skills and songwriting ability. It's violence, destruction, and death, but it's graceful and beautiful.