1. Alex Lifeson - simply awe inspiring ability. Rush introduced me on to prog rock and remains my favorite group.
2. Frank Zappa - virtuoso skill combined with an unequaled imagination and compositional abilities.
3. Martin Barre - I just really like his style of guitar playing.
4. David Gilmour - It's Pink Floyd, what's not to love about it?
5. Richard Williams - I've rated him this high because Steve-cubed (Howe, Morse, Hackett) have already been mentioned and I like his style just as much as I like thiers
Honorable Mention:
Steve Vai - probably the best guitar player on the planet (way better than Clapton/Townsend, chuckle) and I would have rated him number two except he isn't primarily a prog rock artist.