Definitely season 1. That was the only season where I actually had respect for him. In season 1, he was brave, extremely smart, and always looked out for, helped, and defended his friends. In later seasons, though, he became extremely disrespectful, rude, arrogant, overconfident, and was a terrible influence on Eleven.
In season 3 he was at his worst imo.
It was honestly sad for me to see a character who I had admired so much to just plummet.
Incidentally, I felt the exact opposite way with Steve. In season 1, I hated him (I remember hoping he would die!), but in later seasons he became much better.