what was his best movie ?

I'm no scholar when it comes to R Scott, but I've seen him in a few movies and he has a great presence. I remember him in some old film with John Wayne called Pittsburgh. They were steel workers and figured they were smart enough to do better than their owners -- and they were ! They had some romantic complications, (Barbara Stanwyck?), but overall it was a light and fun early film, likely from Republic Pictures (remember the Eagle?).
And Scott made many, many movies other than that. This is just the one I remember most.

My favorite RS films...

1. The Tall T.
2. Ride Lonesome
3. Tall man riding
4. Ride the high country
5. Commanche station
6. Seven men from now.

I like the Bud Boetticher films with the screen plays by Burt Kennedy. 4 of those were in my top 6.

What sets the Tall T apart is that it starts out as a comedy and then becomes incredibly dark. There is a little homosexuality worked in through Richard Boone's character. Some might disagree. It is a great film.

Tall man riding is a hidden gem about a man who is forced to leave town by his fiancé's father and makes a life for himself elsewhere only to return for revenge.

Probably 'RIDE THE HIGH COUNTRY'.