The most memorable episodes for me are from season2, Soul Survivor, Knightmares, Race for Life, Big Iron, CustomKITT, Ring of Fire, Silent Knight, A Good Knight's Work, etc. That's why I voted Season2.
After that, season1 with The Pilot, Trust Doesn't Rust, Chariot of Gold, Forget Me Not, A Good Day at White Rock, etc.
Season3 had also excellent episodes like KITTvsKARR, Lost Knight, Junk Yard Dog.
4th season is the last in my rank, some great episodes here as well like KillerKITT and Scent of Roses.