Brave New World. Simply because that's the album with the most great songs. I think The Mercenary is the only song on the album that's not great. It's good, with a great opening-riff and all, but I think the chorus is a little repetive. To much show them no fear/pain. Also, The Nomad is a great song, but I think it's too long. 7 minutes would've been enough. Other than that, the album is almost perfect.
Then we have Somewhere In Time on second, followed by Number Of The Beast, Powerslave and Seventh Son on a tied third. I'm not sure if I'll put Dance Of Death on third as well, but at least it's just behind. Not sure after that, but No Prayer is the least good album. It's not a bad album, hell no, but one album have to be the worst anyway. Virtual XI is the only album I haven't heard.