I am a very big fan of the Swiss founder of analytical psychology, Carl Jung. Also the Myers Briggs typology and Freud but to a lesser extent. I consider Jung's process of individuation, breakdown of the mind into the fundamental cognitive functions, and theories about anima/animus and the collective unconscious to be extremely useful for self-development and connecting to a wide variety of people. I find Freud's ideas about the Superego and Id also particularly noteworthy.