For me it is the early stuff and nothing, nothing, will exceed “Fall On Me”, just the harmonies, and Mike Mills backing vocals’ to Michael Stipe.
Second up, from the same album IIRC is “I Believe”. It is just pure energy, rambunctionus and has a constant drive.
Third is ‘Carnival of Sorts (Boxcar) from their first album. If you only ever listen to one REM song, that tells you all you need to know about them. Not as pretty as ‘Fall On Me’ though.
The stuff from ‘Green’ is excellent and I would be hard put to diminish the tracks on that. It was how I first heard REM and fell in love with their music but ‘Life’s Rich Pageant’ quickly became my favourite.