flow
Audiophile/Insomniac
This is a tough one, I'm sure. I'm voting for the severely underrated 'Amnesiac'. I've listened through it twice just today, and I'm in love. Second place is tied between OK Computer, Kid A, In Rainbows, and The Bends... yeah, I'm not very good at rating Radiohead albums. They're all so damn good! Oh and Hail to the Thief ain't bad either. 
Edit: I'm sure my opinion of their best album will inevitably fluctuate.
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Edit: I'm sure my opinion of their best album will inevitably fluctuate.