I'll only comment on Adrian and Dave, because I have studied their work extenisively for years now.
-Dave is a truly unique player. He is in my opinion, the only player who can play that fast, and still make a solo mean something. There are many more technically sound guitar players out there (one plays about 5 feet to his left every night), but Dave's playing has this ability to tug at your heartstrings and engage you whilst you're there with him at a gig, and I don't think the other two do that.
Dave never really does anything complicated in terms of the notes he picks or the way he phrases them, which and I guess is why you don't see him in these "Top # Guitarist" lists; he hasn't really changed the way we play guitar too much, certainly not to the extent, Hendrix, Clapton, Page or Blackmore have. Another thing that bugs me about Dave is that I once read in an interview that between tours, he never plays guitar, meaning he has gone several months without playing in the past. I think this is further evidenced by the fact that he is only a little better nowadays than he was when he was 22 years old, (although at Sonisphere the other week, he was as good as I have ever seen him).
-Adrian in my opinion is one of the best guitar players in the world. He is the quintissential rock guitar player, because he can do it all. He can play proper epic 70s rock licks (to perfection), he can play the sweet 60s electric blues licks (to perfection), and he can play more modern stuff like tapping, and can shred like a beast (though obviously not on the level of say, a Satriani or Vai type player). He has written one some of the most memorable Maiden riffs (2 Minutes being the most obvious), and from what I can tell, the most accomplished rhythm guitarist in the band too.
The difference between him and Dave is that on the Killers tour, Adrian was a very basic player, and his since developed into the brilliant guitarist we see today.
There is an intangible quality to Adrians playing that I find hard to describe. Bruce makes reference to it on the Flight 666 DVD with the "pulling notes from thin air" comment, but I've always preferred to call it the "Gilmour Quality". I've found that David Gilmour has the ability to make every single note he plays (and arguably more importantly, the notes he doesn't play) mean as much as possible, and few guitarists can do it like Gilmour. Adrian I feel is one of them. There is an odd timing to his playing which sets him apart, he kind of falls out of pace with the song sometimes, weave in and out of the rhythm, and neatly come back in at the end. I've always said that of Maiden's trio, Adrian's solos are the easiest to play note-for-note, but by far the hardest to play properly. I have never seen someone play the Wasted Years or Wicker Man solos properly, for example.
I think if you take all things into consideration (technique, individuality, expression, songwriting, longevity), Adrian ticks as many boxes as any other guitar player around. I think the reason why he (or Dave) don't top these polls is because they lack a certain "it" factor that the likes of Hendrix or Slash have, and musicians need to have if they want to transcend the genre, and stand the tests of time. I think Adrian and Dave are just normal blokes, even after all their success; they aren't larger than life characters, and never quite captured the imagination of the masses.