I couldn't really give a least favorite, I tended to shove them out of my mind as soon as I'm done with their class.
My most favorite, though...I'd definitely say would be my french and latin teacher in high school. I ended up having her several times, once in my junior year and twice in my senior year (in the same semester, too, due to how they did scheduling; the first day I was in her class, she asked me if I was "sadistic or masochistic?" because of that, lol). Not only was she a nice teacher to be around, but I always thought she was rather cool, too - when we'd have lulls in the class, she would often tell us stories, sometimes relevant, sometimes not, about things that happened to her when she was younger and growing up in foreign countries.
She also accidentally taught me a swear word in french, though I didn't realize it until some while later. But it was in reference to looking over the school reading list for literature class, which I hated too, so. :D Points there.
What would I tell her now if I ever saw her...probably "hi", and then maybe see about catching up on what's happened with her since I left high school. I actually do miss having her as a teacher, and she's probably the only one I miss.