Re The Beatles, I agree that most of their versions of their own songs would be called the definitive versions. One exception I can think of is where the Rolling Stones' version of I Wanna Be Your Man (45 rpm single) was widely considered at the time to be better than the Beatles' own version sung by Ringo on their 2nd LP. I guess that might make the Stones' version 'definitive', but it's pretty much a throwaway song anyway, IMO (I believe they gave it to the Stones and probably put their own version on the LP as a filler).

Some people might consider Joe Cocker's no. 1 hit version of With A Little Help From My Friends better than the Beatles' version (again sung by Ringo - I'm not having a go at him, honest!) notwithstanding it was on their ground-breaking Sgt Pepper album.

But it's all subjective, innit.