Hard to say. I've had some really good road rides both locally and on RAGBRAI. My definition of "best" will vary with my mood from day to day.
Some days I'll tell you it's a ride where a couple of us are going slow and having a good converstion. Other days I'll recall a 27mph paceline through the Iowa cornfields or breakin' the sound barrier on a tandem.
I think right now it's a tie between the day Brent, Scott, & I took a wrong turn on RAGBRAI and went through Backbone State Park and found a road with brand new pavement and a perfect tailwind and the hammerfest with Doug, James, and Greg to Jamesport in a really stiff wind.
I have a good time on lots of rides though. I wouldn't take much to talk me into some other ride as a favorite.