I wouldn't rank it as their best. To be frankly honest, besides Pablo Honey, it's their worst album. That being said, King of Limbs isn't a bad album as such (though I'm reluctant to call it an album - I don't consider anything below ten songs an album, which sounds snobby admittedly but still), it's good. It's just that their albums from The Bends onwards through to now with AMSP that are so far and above superior to what King of Limbs did.
Like I got into Radiohead two years either side of Hail to the Thief and In Rainbows. I was fourteen and this was 2005 and I started listening to one or two songs of theirs before breaking into the album stuff and thus the less known stuff like In Limbo, Disappear and all that. I bought Kid A for myself just before Christmas that year and I knew in my heart of hearts it was mindblowingly special to me in a family and life where I'd surrounded myself my electronica and dance music (but less into the latter) that it was my favourite album ever.
But AMSP speaks to me just as much as any other album because I love all of the Radiohead albums (besides Pablo) in their own way. However, ASMP has a mix of Kid A, In Rainbows, OK Computer - my three most loved Radiohead albums in that order - as well as that heavy usage of an orchestra in this one which just makes so much sense thanks to Jonny's incredible work.
So yeah.
TL;DR - King of Limbs is their worst album, if you don't count Pablo Honey, but that doesn't necessarily mean it's bad.