The last King of Scotland, the man who played Idi Amin was Forest Whitaker. A brilliant actor who in my opinion is long over due for recognition. (Lets just forget Battlefield Earth).

The Shield
The Crying Game
Ghost Dog
Platoon

Honorable Mention - Good Morning Vietnam

Fast Times at Ridgemont High was a fun film but his performance didn't stand out for me.

Is Forest Whitaker a Scientologist?