The theory which has long lasting effect is definitely by Sigmund Freud.He laid the foundation of personality theories. Further more, he put forth many important concepts like consciousness, un consciousness, defence mechanism. Later, Neo Freudians carried on research.

The question as to which theory is widely used.... Each personality theory is built on different assumptions and premises and focuses on some unique pattern of personality. Furthermore, there is no scientific evidence which supports any single theory as the best one.
Freuds theory, though very significant, is not scientifically proven.
Traits theories focus on personality traits only. No mention of other aspects of personality.
Learning approaches focus on external factors that shape personality and largely ignore internal once. However, they are widely used to treat Psychological disorders along with pharmacological treatment.