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He is most recognizable as villain Curly Bill Brocius in the magnificent western Tombstone, as the nemesis of the Earp brothers.  In that role, IMDB calls him "oddly likeable", which is true, and quite a tribute to his acting ability.  Boothe also played several western tough-guy roles, including barkeep Cy Tolliver in the acclaimed Deadwood series on HBO.

For my money, much as I loved him in Tombstone, Powers Boothe's best role was one of his first.  He played Guyana "People's Temple" cult leader Jim Jones in the 1980 television docu-drama Guyana Tragedy, the Story of Jim Jones,  the story of the mass suicide of more than 900 people (and murder of Congressman Leo Ryan) in the remote commune of Jonestown in 1978. 

Boothe's powerful presence and deep voice were features of his ability to play disturbing bad guys.  However, unlike a lot of Hollywood, and unlike most of his characters, Boothe was reportedly one of the good guys.  He grew up working in the fields in Texas, and was described as a gentleman by those he worked with.  He will be missed.  (URR here.)