The Beast from 20,000 Fathoms - the beast grabs a cop by the head, tosses him in the air and swallows him. There's a similar scene in Jurassic Park, but this one came 40 years earlier and IMHO the scene in JP is an homage to this one.
Bonnie and Clyde - the wages of sin is to be machine gunned by Texas Rangers.
Conan the Barbarian - Schwarzenegger decapitates James Earl Jones.
Dr. Strangelove - the pilot rides the H bomb.
The Godfather - Sonny ambushed at the freeway.
Jaws - the opening scene where the shark eats the girl. This one has more impact than the one where the shark eats the boat captain because by then we've seen several people get eaten.
Lawrence of Arabia - the execution of Gasim.
Once Upon a Time in the West - the bad guy shoots a child.
Psycho - the shower scene.
Raiders of the Lost Ark - the villain explodes.
Star Wars - Darth Vader kills Obi Wan Kenobi - or does he?
War of the Worlds(1953) - Three guys wave a white flag at the martians and get cremated for their efforts.
White Heat - James Cagney "Made it Ma! Top of the world!"
King Kong, Platoon and Butch Cassidy and The Sundance Kid are particularly good additions to the list.
Midnight Cowboy is a good choice because it is dramatic, whereas most of the others are sensational. From my own list Lawrence of Arabia is the only dramatic one. I think there is a movie called Camille, which I have never seen, that a lot of people would include. Other dramatic rather than sensational scenes I would include:
'Great' may be the wrong expression for it, but there's a hilarious death scene in Charlie Chaplin's 1916 short 'Burlesque on Carmen' where Charlie kills Carmen and then himself (or does he?!).
The Exterminator (not to be confused with Cameron's film) -- the famous decapitation scene of the GI by the Viet Cong
Love and Bullets (1979) - an obscure (and not that good) Charles Bronson film. The ending scene showing him driving off as a "coffin bomb" detonates in Rod Steiger's mansion behind him was the best part of the movie (and the only part where Bronson actually smiles)
Apocalypse Now - loved the editing on the Willard-killing-Kurtz scene
El Cid - the final battle when Charlton Heston's body is propped up on his horse and he ends up killing the lead villain
Death Wish II -- the famous "Do You Believe in Jesus" scene...