Monster has been a grower for me, but I think it's pretty solid now. Probably still a handful of REM records I'd take over it, but it's an enjoyable listen and, I think, despite the narrative that it's supposed to be a return to rock after Out Of Time/Automatic, fairly experimental for them.
I'd say it has two top-shelf tracks that I'll always come back to. One is the obvious Kenneth, which I think is just one of the most killer singles of their career.
The other is Let Me In, which despite sounding like REM doing a MBV imitation, is haunting and genuinely moving to me. It transports me. Honestly it might a career highlight for me as well. I love it.
Other favorites that don't quite hit the heights of the aforementioned tracks are Star 69(relentlessly catchy), Strange Currencies(even if it does sound like an attempt to recreate Everybody Hurts), Circus Envy, and You(I love the way this one just builds and builds and climaxes with the high vocals, quiets down, and then starts all over again).
I Don't Sleep, I Dream has been growing one me too.
Even Tongue is an interesting experiment, with the falsetto vocal and the organ. I think that one gets crapped on sometimes, but I like it.
It's a really enjoyable album and caps off what I think is one of the strongest five-album runs in recent memory - Document/Green/Out Of Time/Automatic/Monster.