I think any session where I feel we've got somewhere, made some progress, or I feel better afterwards. Some sessions don't always feel so epic or 'successful' at the time, but later on, upon reflection, they've still proven important as part of the 'bigger picture', if you know what I mean. Just my interpretation, I imagine the answer will be different for everyone? I'm not sure what my T would consider makes a session a 'success' or not...