Gene Hackman seem to be one most versatile actor in the business as he is multidimensional. Having a along resume, it was tough to narrow it down to only three favorites.
The French Connection was turning point in his career, so will be labeled as The Honorable Mention.

Not in any particular order:

The Poseidon Adventure (1972): Why see the remake when we can watch the original with Hackman as Rev. Frank Scott.


Hoosier (1986): I wished Dennis Hopper has more role in this movie as he steal every scenes he is in.


Crimson Tide (1995): This is probably the most intelligent movie from the dynamic trio (director Tony Scott and producers Don Simpson and Jerry Bruckheimer), and one of better Submarine movies.