I voted for 'Stairway to Heaven'. Most of the other solos are technically more impressive and feature more out-of-the-ordinary moments in them. But this almost strictly pentatonic solo is just my favorite. It sounds like life, like humans, like a story, just natural. It sounds completely effortless. Jimmy Page says that the solo was largely improvised.. incredible.
Close second for me goes to 'Blackbird'. Definately the best traditional rock solos I've heard this decade, and most certainly memorable.
At least an honorable mention to 'Comfortably Numb'. I really think the solo on PULSE is epic, but I'm just not that into the Floyd at the moment. It's my favorite Gimour solo though.