One of the major problems with sentiment analysis is that it doesn't pick up on irony or subtext very well. Relying on word choice alone, one such analysis pinned "Fitter Happier" as one of the happiest Radiohead songs.

http://rcharlie.com/2017-02-16-fitteR-happieR/

Edit: sorry, I misremembered - it actually handled "True Love Waits" pretty well. It did pick up "Fitter Happier" as one of the happiest, that's what struck me as strange.