The Big One was my TransAm tour. Big in many ways: time, effort, resources. Just seeing so much more of the country than I'd ever seen before -- people, places, scenery -- made it special.
A couple of week-long tours in Wisconsin get honorable mention. After suffering through 4-8 weeks of southern heat and humidity, a week where the natives complain about temps in the 80s was a break that got me re-enthused about riding through the summer. (We've had a month where the temperature didn't drop below 80F, with dew and 100% humidity 2/3 of that period in the morning!)