I track the standard stuff, pace, cadence, HR, elevation, GPS track, but think you want to also remember to capture how you feel during any given workout. Capturing objective data like pace and HR is easy because it’s done by your phone/watch/whatever, but being able to combine that with more subjective things it will give you more insight into how you are responding to your training.
On TrainingPeaks for example, my GPS and HR data is uploaded automatically from Garmin, and I then add a comment about how I felt as soon as possible after completing the workout.