Litespeed titanium. I got into cycling in the early 90s, and titanium was the unobtainable wondermetal. I couldn't afford much of anything back then, but a decade and a half later I could.
It was everything I could have imagined - responsive, comfortable, and timeless.
I like it so much, that I'm currently building a second titanium ride, but a Lynskey (father of Litespeed) this time around.