Wouldn't mind having my Superstar back. It was some weird pre-production model that I got cheap of some old BG person, and had a super short backend (for the time). It just ridiculously dented up and I couldn't trust it anymore.
Wouldn't mind having my PW moto back, too. Not because it was my best riding bike ever, but i learned a ton of trick on it, and was around for a very important time in my life. Also, it is the only time I've gone out and purchased pretty much an entire new bike at once. (though my pre-megatour purchase spree was almost on that level)
Lastly my 125R. I raced on it for 3 years, had a ton of ups and downs on it, and loved the way it rode.