I'm going to start with a big long list and maybe in another post, highlight a few songs and give reasons.
my list, by albums and the accompanying b-sides, side projects, etc. era:
Preface
There are several factors in my list here. One is the exclusion of the cathartic song. There are several songs that could leave one on a purifying emotional note, but the exhaustion rules out 'happy' as too shallow or inaccurate a description. I approached this partially as a list presented to someone who starts with the notion that Radiohead is doom and gloom and never hopeful.
So, songs like There There, Fake Plastic Trees, Stupid Car haven't made my cut. Some of these are cathartic, but not as disqualifyingly draining.
A simpler factor is the hopeful song that deals with an angry topic (typical something like the failure of world leaders, greed of corporations) and it makes the cut due to hopefulness (Go to Sleep), but some songs relish in the anger and don't make the cut (2+2=5). I think I prefer the 2+2=5 approach.
Last, a number of songs at first are neutral, and an analysis of lyrics and music tip them over to one side or the other.
So... here we have mine:
Lurgee
India Rubber
I Promise
Lift
Airbag
Let Down
Karma Police
Meeting in the Aisles
Everything In Its Right Place
Kid A
I Might Be Wrong
Cuttooth
True Love Waits
Fog
Go to Sleep
Sail to the Moon
Atoms for Peace
Iluvya
Hearing Damage
Open the Floodgates
All for the Best (granted it's a cover)
Reckoner
Bloom
Lotus Flower
Separator
Supercollider
to highlight songs from my list that are obviously and immediately happy songs, where there's no doubt that the lyrics or music might have a sarcastic edge, avoids heavy themes, or at least deals with them lightly enough:
I Promise
Lift
Cuttooth
Worrywort
All for the Best (granted it's a cover)
Separator
Supercollider
Interesting to note how little The Bends and IR eras show up here, given that they're more accessible, whatever you want to say, lighter, than most of the other eras. The Amnesiac b-sides are pretty strong.
If I'm picking only one (which is the point here), I'll take Worrywort. It's an understated song, but ridiculously and wonderfully happy.