My absolute favorite was my 6th grade teacher. She basically helped me to stop underachieving/work to my full potential, as my self confidence wasn't high. Her class was a lot of fun.
Since then I'd definitely say my GT English teacher. [We're like a family in her class (only 10 kids), so we're open without fear that we'll be embarrassed or that it'll get out somehow] She's very insightful and extremely wise, and thus helps us with any problems we might be having, whether it be understanding why a friend could be acting the way they are or just giving us insights on people in general.