I don't know if there's a more popular or more widely-respected Weird West game than Undead Nightmare. And it was a DLC for an existing game. Of course, it's probably so well respected because most Weird West games are from small indie developers yet this DLC was from a AAA developer expanding on an already AAA game. Whatever it was, it worked. It perfectly blended a Wild West setting with a zombie attack.
Here's a trailer. You can play it anywhere Red Dead Redemption is available.
Definitely up there with the best of them.