It is difficult to choose just one album. They are all great with Pablo Honey -- for the exception of a few tracks -- being the least of the greatest.
It's easier to list favorite tracks from each album instead, and even that list can change since I have a tendency to load up one, two, maybe three songs into the playlist and repeat those tracks over and over everytime I listen to music, sometimes repeating them for weeks at a time and listening to nothing else except those specific Radiohead songs.
Current Radiohead song: Fake Plastic Trees, from The Bends