Id have to have Into the Valley by the Skids or the faster bit of Loch Lomond by Runrig.
Best walk on Ive seen was a match between Daryl Fitton and Ted Hankey - the crowd all booed Hankey when he came on to his Dracula type tune and even though Im not a great fan of Fitton - he came bursting on to the theme tune of Hawaii -5-0, and took the piss out of Hankey on the stage. Classic.