Here's a really tough one. Radiohead just so happens to be my favorite band, and probably one of the more musically intelligent ones at that.
Which album is their best?
For me it's a tough call. I love all their albums (Pablo Honey doesn't really count in my opinion, except for Creep which is a great song), from the Bends all the way up to King of The Limbs. Each album was musically adventurous and Thom Yorke has one of the best voices in rock n' roll, or any genre ever.
I'd choose The Bends, OK Computer, Kid A, and In Rainbows. The Bends offers some of the most original and creative rock anthems ever, while OK Computer is the most beautiful and alienating record they've made. Kid A marked a drastic change in their music, but upon further listens it ranked as some of the best electronic music ever. Then we have In Rainbows, which has a nice jazzy rock feel to it.
Like I said, it's really tough, but I'd have to go with OK Computer. Each and every song is a futuristic masterpiece (even Fitter Happier, which sounds like Stephen Hawking when he gets a rap career), and the songwriting is just pure genius. I could say that for every Radiohead album, but this was them at their best when it came to rock music. Simply put, the evolution of this band is amazing. They came from a generic grunge imitating band to amazing rock musicians, and then amazing experimental musicians as well. OK Computer was the album that guaranteed every Radiohead album from then on out was a must buy.