I guess this will turn into a Metallica vs Maiden thread, since they are the only serious contenders.
I feel Maiden has been more consistent in the quality of their output (but also a bit more safe). Their albums after 2000 are good if you are a progressive rock fan.
Metallica took more chances with their sound but they rarely paid off. The metal community are not like Madonna fans, they are very conservative, they would never stand for their favourite band reinventing themselves too radically. I personally like Load and Reload while everyone else hates them. S&M 1 is a fantastic record and a very successful experiment. After 2003 they were either shit or embarrassingly mid. St Anger is a pile of shit but this is more due to the production, the lack of polish and the unnecessary length, not the songs. I've read the Lou reed record is shit. The other albums (Death Magnetic etc) are so mid I can hardly remember anything other than a couple of songs. I heard about two minutes from their last record and never played it again. Hetfield, now in his 60s, still goes banging on about his childhood.