Mmh, difficult to say. I think I had lots of "good years", but for wildly different reasons.
In retrospect, my high school years were a blast: I cruised through classes without much effort, had excellent grades, had a lot of very good friends (some of which I'm still seeing today), drank a lot, partied a lot, and in general I had an hell of a good time. On the other hand, my love life was desolate, and I felt I lacked something in my life.
College years were good. I found the girl of my life, cut down the parties but still had good fun and had a nice social life. Classes were hard, but I did worked hard and get good results.
Grad school years were still good, but just more of the same. Same girlfriend, same friends, but some difficulties at university. Classes were mind-screwing difficult (talking about quantum mechanics and general relativity here) and I struggled to keep the pace. I spent a year working for my master's thesis: getting my master's degree with top marks was probably the most satisfying moment in my life, but I was exhausted.
PhD years were uneven: I had to move to another city continue my career, and it was a sore point. Luckily it was not too far (about two hours and a half commute), but it meant that I could see my girlfriend and friends only on the weekend. Easy to say, I wasn't happy. Work is also very difficult, and I butted heads with my supervisor more than once. Working in a high-level research environment is really exciting, tho. Electrifying, even.
I'm finishing my PhD this year, so I will tell you how post-doc life is in a while.
So, all in all, I would guess that college years were the "best", but I had other good moments, for different reasons.