They are all great artists, and all bring something special to Iron Maiden.
My roommate is a huge defender of Dave Murray, and I'll admit, everything that man plays is so smooth it hurts, but he's not my favorite composer.
Janick has a lot of energy and has composed some killer songs in his time. He catches a lot of flack, but he's a pro and an entertainer. I can't stand when people get huffy about his karate guitar when there's a 10 foot zombie walking around on stage. This is a rock show, have some fun, damnit.
Adrian Smith is on another level. He doesn't move so much, but he makes up for it in every other way possible. First off, he's the only one besides Bruce who can sing in the band (Sorry 'Arry, I know you try), he has in general always been the best composer of the lead guitarists, and his guitar solos are a giant phallus that lords over the other solos so epically. Let's think about it, in general upon first listen you can tell which guy is soloing, but when it's Adrian, you know IMMEDIATELY that it's Adrian. And that's why he's my fave.