It has already been mentioned but definitely Two Mules for Sister Sara with the Morricone sound track.
A bit left field but as a relatively new film and set in a modern era, I enjoyed “No Country for Old Men” and it left me feeling like I was watching a spaghetti western.
Some film critic said Two Mules for Sister Sara (1970) was the most spaghetti-esque of all US-produced westerns. It doesn’t have the operatic style of spaghettis, but lots of catholic stuff, Shirley MacLaine dressed as a nun, and so on. Shot in the real Mexico, not in the ersatz-Mexico of Almeria though. Morricone’s main theme incorporates some Gregorian chant (plus the sound of a donkey).
Joe kidd from 1972 is a film that definetly feels like spaghetti although it ovbiously isnt since its American but i really liked that film Joe kidd is underrated.
2 Mules for Sister Sara has some SW vibes, no wonder with Eastwood in SW mode and a fine Spag-like Morricone score, but Joe Kidd seems to me very typical for a 70s US western.
High Plains Drifter (Eastwood, 1973), El Topo (Jodorowsky, 1970) , Sukiyaki Western Django (Miike, 2007) and to at least some degree The Proposition (Hillcoat, 2005) have a bit of a spag aura about them for one reason or another. More contemporarily, Machete (Rodriguez, 2010), Once Upon a Time in Mexico (Rodriguez, 2003) and even the bizarre Rubber (Dupieux, 2010) all feel a bit spaggy to me; again, in different ways.
And it may well be a sci-fi series spun off of a multi-billion dollar franchise, but Jon Favreau’s The Mandalorian (2019-present) is a spag in all but name.
Merely IMHO, of course.
Most 70s ones are obvious rips of them, notably Barquero, with Van Cleef (for Heaven’s sake!), but chock-full of the American West stock company, like Warren Oates and Forrest Tucker.
An underrated John Wayne classic is The Train Robbers, where the opening credits has Ben Johnson and a bunch of other guys waiting for a train in the dust and wind, with no music. It was directed by the great Burt Kennedy, and an obvious Leone tribute.
I don’t remember getting spaghetti vibes. I think Two Mules for Sister Sara would count. Hang ‘Em High has a few scenes at the beginning and High Plains Drifter is definitely Eastwood’s own tribute to Leone.
Valdez is Coming… maybe a bit. Still feels very American with the exception of the Spanish locations though.
The Last Rebel (1971) is an American western filmed in Italy. It looks and feels like a spaghetti western.
I always assumed the goofiness of The War Wagon was an intended Spaghetti copy. But they may just be me. That’s what I thought when I first saw it back in '21.
The War Wagon is from 1966, the Leone films were not released before 1967 in the USA, and I think other Spags had barely any effect on US westerns.
I agree with @stanton I can’t see any spaghetti influence in the overwhelming majority of American westerns.