Long rides and short rides. Supported rides and unsupported rides. Nature focused and culture focused. One of the things I enjoy about touring is the sheer variety and outlook in some of the different rides I have done as well as "newness" and adventure of doing something different.

Hence, rather than a single "favorite", I'll give you three different categories and my "favorite" for each:

Favorite long unsupported ride: Around Australia in 2001. 8 months and 19411 kilometers.
What made it special: Travels were not too difficult and I spoke the language. I got a real sense of the country and particularly enjoyed my time camping in the outback areas and talking with other campers. I also had a sense of "completeness" in visiting all over as well as a sense of accomplishment in making a full lap.

Favorite short unsupported ride: the Klondike loop from Haines, Alaska to Skagway, Alaska by way of Haines Junction and Whitehorse.
What made it special: Awesome scenery and remote mountain vistas. The physical challenge in crossing the coastal ranges as well as good camping along the way. Interesting contrasts between towns I traveled through. A bit of historical tie to older Klondike gold rush. Good weather the week I went.

Favorite supported ride: A Spice Roads trip I took in Thailand and Cambodia.
What made it special: Cultural experiences cycling through back roads in Cambodia and a sense of how locals live - away from the tourist zones. Angor Wat and impressive temple complexes. Good logistics and organization. Reasonable fellow tourists as travel companions.