I thought Braveheart was quite a good movie including both his acting and directing.
Mel's acting in Gallipoli and The Year of Living Dangerously (admittedly older movies) prove that he is quite capable of being a good actor in the right vehicle. Also liked him in Ransom, Conspiracy Theory, The Man without a Face, and Signs.
Ransom was a lousy movie. Signs was a nutty movie.