There is never any obligation to give year-end teacher gifts. However, if the teacher has done something (anything!) good that you would like to encourage, there may still be some value in a small thank-you of some kind, even if it is just a nice card from your son, as positive reinforcement for the good elements, however few. Plus, it may help demonstrate that any animosity in the relationship is not on your side, or at least that the relationship was not wholly adversarial, which may or may not have value to you.
The main complication is if the two teachers are in the same school, or otherwise likely to find out what the other received as a gift from you.
In the past, some of the things I have given include small-ish ($10 range) gift cards to easy-to-use places like big-box bookstores, Target, Walmart, local-coffee-chain-of-your-choice; child-made thank-you notes; small food item (this depends a lot on what you know about the teacher and their food situation); small fun bath item (lotions, scented hand sanitizer, etc.); pretty stationery items (fancy pens, decorative paper).
Most teachers I know very much appreciate a heartfelt card directly from the student, articulating something specific that was positive from the year.
And for your great teacher, FYI: most states forbid PS teachers from accepting cumulative gifts of greater than $50-worth of monetary value per school year. Some have even lower limits.
