WKO+. I watch my TSB, and see how its data matches up with how I feel (rested, tired, etc) during periods of intense training. I use the graphs to see watts / heartrate over time as one measure of fitness. I also keep track of watts/kg over time of course, always hoping for that upward trend.

About 20% of my rides go to Strava....anything with climbing. Everything also goes to garmin connect just because it's easy.