While there are certainly cases where it is true, it is not always the case where future versions of a song are "better" than others because they were recorded in the same genre (i.e. country music), as opposed to versions which cross over to other genres. I believe "C-H-R-I-S-T-M-A-S" falls into that category.

"C-H-R-I-S-T-M-A-S" is not a very "country-ish" song - musically or lyrically- whether it be Eddy Arnold or Jim Reeves' versions. Kitty Wells' version is, however, a country-sounding recording. I believe an artist could record this particular song in a straightforward "pop" form, like Perry Como did, and it wouldn't sound the lesser for it.

Eddy Arnold originally recorded the song in 1949, but it didn't appear on his Christmas album until 1962, the same year Kitty released her Christmas album; Jim Reeves' was released in 1963. I believe Perry Como recorded and released (on album) the song several years earlier. So, for awhile anyway, Perry's version might've been considered the definitive version.

On a message board, it is obviously not necessary to state "In my opinion" when talking about songs, albums, and performances; it is assumed. So, when you say that the "best" versions are the country versions, that is your opinion. I really like Eddy Arnold's version; Jim Reeves' is also good; I'm not crazy about Kitty's take. I still think I PREFER Perry Como's version.