That's a hard question to answer.. one man's 'best' will be another's worst nightmare.
UK: sites are good but too expensive, but has a great selection of CL sites.
France and Germany: the most motorhome friendly with their network of aires, municipals and stellplatz.
Spain: has some great sites but they tend to be crowded in high season and again expensive and has few 'aires'.
Best for what? Partying all night, a bit of peace & quiet, or near to places of interest. Cleanliness, Friendliest, best price. Sites are just too different to ask which countries are "best".
I have stopped on remote mountain Spanish sites in the mountains where the only life seemed to be the soaring birds of prey above, by contrast I have been on Spanish sites where families sang and played football till 2am. I have been on British sites where I have literally wanted to murder the warden, and others who were so nice it was almost uncomfortable. I have been on French sites where the owners have been rude and racist, and others where they have bent over backwards to ensure we enjoyed our stay.
If you asked which country was most sympathetic to the needs of the motorhomer, I would say...