Any ride when I finish feeling energized rather than exhausted, and better the next day rather than worse.
For me, due to multiple physical and health issues, that varies greatly from day to day. Some days, about all I can manage is the 6 mile round trip to and from physical therapy. Other days, 50-60 miles feels pretty good as long as I stay within my limits.
The problem is that when I stayed well within my comfort zone, my conditioning didn't progress. I had to risk feeling bad in the short term to feel better overall.
I tend to push myself beyond my comfort zone pretty often. So while it helps enhance my conditioning in the long run, making some challenges easier -- such as climbing short, steep hills -- in the short run I have a lot of miserable day-after recovery days... sometimes two recovery days before I'm ready to ride again.