I just love album Piece of Mind. It is first cd I've ever bought. They did mixing very well: everything fits together and you can here the bass well, guitars don't steal the show except in solos where they are meant to and drums sound nice (Nicko is such a cool drummer).
The best song is Flight of Icarus. It's so original Iron Maiden stuff. Creative bassline, simple but not too simple guitars, awesome guitar solos, nice drumming and amazing lyrics. Second best would be Run to the Hills.