My favorite is without question In Rainbows. I also like Hail to the Thief a lot, but I'm not sure if I'd rank it as my second favorite album or not. Amnesiac is good as well, but I haven't heard it in at least a year so all I remember is that I don't think it's as good as Hail to the Thief. The King of Limbs I needed to hear several times before I started appreciating it, but it grew on me and now I like it a lot; not as much as In Rainbows, but still a great deal.
What do I think of OK Computer and Kid A (and The Bends)? Well… they are "nice", I guess. I think I've rated OK Computer 4.5 stars on Rate Your Music so I do think it's a very good album. I just almost never want to listen to it so perhaps it lacks something I look for in music (don't know what that is, so don't ask). Kid A… I've never learned to know well enough to have an educated opinion on it. The Bends has some songs that I really like but I haven't heard it as an album in a couple of years so I can't remember how well it works as a whole. I'll have to revisit all three albums at some point, but right now I'm not really in a Radiohead state of mind.