If you discount my 1980 back-to-school, planned-only-the-night-before 200 mile (13 hour) escapade on my 1978 Raleigh Super Course, it has to be the three Seagull Centuries on the trusty, 1972 Fuji Finest. Apparently, I never moved the photos I took of it at the turn-around at Assateague from the defunct Photobucket account, but here's a photo of it lacking only the bags it carried - though not on the day-of.
The bicycle has the distinction of simultaneously being one that I could consider parting with, and also being what I think I would keep if I could only have one bicycle.