Wow! Very hard to choose between S2 and S3. I have never loved the bad boy before (another RA first for me). I was so relieved when he finally joined the good guys at the end (not to mention his new cleaned up look was gorgeous!) But hey, he was even gorgeous filthy and desperate! But I so loved his interaction with Marian (except of course for the last episode), that I had to pick S2!
I started really loving Guy in the middle of the first season. I think it was Episode 6, when he insists that Marian wed him to avoid prosecution by the Sheriff. His expression was what got me. He went from evil (You're dead already!) to regretful and compassionate (As my wife you will have protection) all in the blink of an eye. It's Richard's performance, of course, that captivated me, and kept me wondering...Marian, what the heck is WRONG with you? You would choose a boy over a MAN? You need someone to knock some sense into you.
Sorry if I rambled here. Guy of Gisborne does that to me. :D
S3 proves that Marian, grrr, was right. There was a better man inside there. A, like many of us, he had to hit the nadir of his life to be able to change. Love alone wasn't strong enough, love and grief and the abandonment of hope was strong enough for him to change.