Is it weird I've always thought that Hail to the Thief would make a good introduction album?? You've got more cerebral, yet still raucous songs ("2+2=5"), the electronic experimentation ("Backdrifts" and "The Gloaming"), the big epic crowd pleasers ("There There"), quiet and beautiful ballads ("Sail to the Moon", "Scatterbrain" and "I Will)", more straightforward rock songs ("Go to Sleep") and others that blur the line between all of these subcategories.
To me, it's a very diverse album that gives you every shade of Radiohead you need to know about.