Yeah, Murmur is great and all, but I just love the harmonies on Reckoning. "Seven Chinese Brothers" and "Harborcoat" are just divine. Everyone should own Reckoning, I think.
I really like Monster, too, but I don't think it quite makes the short list of "R.E.M. albums everyone should own." It's not as consistently good as either New Adventures in Hi-Fi or Automatic for the People, in my opinion. "What's the Frequency, Kenneth?" is one rocking track though.
My personal favorite R.E.M. album has changed quite often over time, but Lifes Rich Pageant was probably my favorite for the longest total amount of time.
My list of R.E.M.'s best albums:
Murmur
Reckoning
Document
Lifes Rich Pageant
Automatic for the People
New Adventures in Hi-Fi