If I absolutely had to pick, I'd have to select OK COMPUTER as my favorite. However, Kid A, Hail to the Thief, and A Moon Shaped Pool also are in constant rotation on my car's stereo.
The honest truth is that Radiohead hasn't made a bad album. I suppose that the general consensus is that The King of Limbs is the most, shall we say, resistible, but even that album has some true gems. The song "Codex," for example, is one of my favorites of all their songs.