Wild Bunch takes this one for me pretty easily. It's admittedly one of my favorite Westerns. Pike Bishop was a pretty cool character, and it doesn't hurt that this movie has one of the best ending shootouts that I can recall.
High Noon... well, um, it has Grace Kelly in it. ;D Seriously, I'm not a fan of that film, sorry.
High Noon for me. It's a better character piece and ratchets the tension up superbly as time advances towards noon. The hero has everything on the line - life & the love of his new bride - and risks it all to make a stand against terrible odds because of the consequences for his town if he doesn't.
Wild Bunch is a good film, just not in that league.