Seeing Dorian Yates at 300lbs offseason walking round Temple Gym back in '96 was probably the sight that stands out for me the most - probably because I'd never seen an IFBB pro bodybuilder in real life up to that point.
Although I've got to say seeing Joel Stubbs and Ronnie Coleman at the 2008 Olympia Expo was pretty impressive too considering I'd seen lots of pros up to that point.