If I could only pick one, it could be Powerslave, because it really was an interesting step up into a great direction. But I also hold Fear of the Dark high in regard for its innovation and variation. Somewhere in Time and Seventh Son are absolutely top notch for the melodies and the playing. The X-Factor, often attacked for its production and songwriting, has without a doubt very nice lead work (and some neat riffing), and it's the only studio album on which you hear all solos on the left and right channel.
Iron Maiden: Great riffs, lead work and clean nuanced playing (e.g. Charlotte the Harlot, Remember Tomorrow, Strange World) .
Killers: an album full of exciting riffs and great lead melodies and solos. Prodigal Son is an absolute highlight of the discography in this department. As is Genghis Khan's "Hallowed Be Thy Name" section. Purgatory has menacing guitar work as well.
Piece of Mind: full sounding rhythm guitars, lovely lead guitar work and production. Adrian does some great melodic stuff.
Powerslave: first album with more detailed and varied guitar work, rhythm guitar wise. Different things happen on the left and right side, e.g. Adrian's long chords and Dave's fast riffing in The Duellists or Rime.
Somewhere in Time: a continuation of what happened on Powerslave, the solos are produced and played very well, and Adrian plays his best stuff. Very catchy guitar melodies on this album.
Seventh Son of a Seventh Son: so many good guitar melodies and riffs. What a feast.
Fear of the Dark: Dave plays some of his finest solos. In top form. He does astounging solos in the title track and Judas Be My Guide among many others. Some of his solos are more worked out, well prepared. Weekend Warrior is a very good example of this.
Very good lead guitar production as well. The album brings acoustic guitars. Maiden tries out new things, interesting variation.
The X-factor and Virtual XI: Janick plays some of his best guitar solos (e.g. The Unbeliever, The Clansman). Blood on the World's Hands is an evil riff feast, pretty unique stuff. \m/
Brave New World, A Matter of Life and Death and The Final Frontier: love the different colours the three amigoes have brought in e.g. Brave New World (title track), Dream of Mirrors, Lord of Light and Isle of Avalon. Ghost of the Navigator has a superb Janick Gers solo. His in The Pilgrim is up there as well.
The Book of Souls: lovely lead work (The Book of Souls, The Man of Sorrows, Tears of a Clown)