Depends on which circles you frequent - I think many of a slightly younger ilk (and by that I mean 20s-30s, rather than 40s+) see In Rainbows as their best album.
rateyourmusic.com has it as the 4th greatest album of all time (1st - Kendrick Lamar - To Pimp A Butterfly, 2nd - Radiohead - OK...
Great question. I'm fairly populist with my favourite RH songs (Let Down, Idioteque, most of In Rainbows).
I would say Present Tense is possibly the most beautiful song they've ever written (a huge call, I know), so I'd say that. I think it's the best thing they've done since In Rainbows.
Oh...