Freebird easy. Allen Collins was unbelievable, Steve Gaines was pretty darn good and Gary Rossington was brilliant also. Collins wrote Freebird though. Stairway to Heaven is every students favourite guitar solo because they know no better. I could put forward numerous guitar solo's better than Stairway to Heaven but I would struggle for Freebird.
My personal favourite solo's are Brothers in Arm's live, Freebird and Oye Como Va.