For me, it was this little beauty:-
I'd been trying to track down a copy for about five or six years and had only ever seen one on ebay and in all my years as a Transformers fan I didn't know anyone who owned a copy! I knew people who owned the Playstation version and loose N64 cartridges (which I had), but a boxed N64 copy was pretty much like gold dust because of the fact that it was a Blockbuster rental exclusive and that the packaging for most of the copies were destroyed by the stores as they just didn't need them!
In the end I managed to get this one at the end of last year when TWO turned up on ebay at the same time! Both were boxed and complete, but the cartridge for one of them had Blockbuster stickers all over it (which are nigh-on-impossible to remove) so I went for the best one but it cost me over $200!!