That's a real tough question. If you're talking anything, any toy line, I'd say the Great White Shark by Mego made in '75. It took me many years to locate this. It's in the middle of the picture near the Starsky and Hutch car.
If your talking about TMNT stuff. It would have to be the costume that I ended up with. Here's the link to the old post.
http://forums.thetechnodrome.com/sho...shells+costume