This is the kind of stuff that I play in my shop if you walked in on me.
Roy and Gene are still the best. Hands down.
From the top, just picking the western ones,
Billy Walker (As in "Cross the Brazos")
Faron Young (As in "Yellow Bandana")
Lorne Greene (As in "Ringo")
Eddie Dean (nearly forgotten move cowboy. Roy Rogers and Gene Autry both declared he had the best voice of the cowboys.)
Smiley Burnette (Gene Autry and Roy's sidekick. He had some hysterical songs)
Jimmie Rodgers (1920's, not the rock and roll nutcase)
Jimmy Wakely (cowboy who tried to fill Roy's shoes on the radio)
Rex Allen (cowboy who tried to fill Roy's shoes on the silver screen)
Marty Robbins (of course. One of my all time favorite voices)
Sons of the Pioneers
Wilf Carter (Canadian cowboy and yodeler)
There's also a million and one small bands that always had some good harmonies and catchy songs.
For those of you who like this stuff, if you have unlimited internet, go to archive.org and search some of these names. Folks have uploaded record rips of these rare songs. Sometimes a couple hundred of each singer. I have a couple gigabytes on my computer of them. Can't get enough.