OK (Corral?)! The quintessential Western must have:
The stench of horses
Cowboys
Bandannas, boots, bullet belts, badges and a lot of alliteration.
A virtuous woman
A soiled Rose
Spurs
Colt .45 Peacemakers
Winchester 30-30 with or without saddle ring
A good guy gone wrong OR
A bad guy gone right
Especially needed, a razor sharp definition OF Right and Wrong
A showdown/climax of blazing guns and dramatic death throes
And a terse summation from the protagonist.

Not entirely essential but would be sorely missed are:

A sidekick.
A saloon with bad whisky, cheap women and gamblin'
A pathetic kid
Indians
Rattlesnakes
A lynchin' or two and concomitant mob
A big sky with a contrail or two!
Conestoga wagons
Wanted posters
Dynamite

Feel free to add to it!