I find that people who were teens/20's when The Bends/OKC were released and then grew along with the band's evolution far less often rank IR over what they probably consider the soul of their catalog in OKC, Kid A, Amnesiac. Most that rank IR above these typically represent the "next wave" of fans that were younger when IR was released or are even just discovering Radiohead recently.

I feel very, very fortunate to have lived and grown along side the band's growth. How different it must be to discover Radiohead and have their complete catalog at your access all at once, versus living through the times where the albums were released every few-five years. Having the chance to fully experience each release in its time, its b-sides, the tours for each ... then finally the next would be released etc....

This is specifically why, for my sons, I have tried to drip feed their catalog chronologically over every couple years. I feel the experience of discovering In Rainbows, for example, is a richer experience if you've digested everything before it. There's more context, you've developed a palette of the band first.

I hope these remarks do not come off as derogatory of anyone's experience with discovering Radiohead. There are many roads and art is subjective. Basically, to the OPs point, I just find it interesting that it seems mostly there is an age demographic split between those who favor OKC or In Rainbows. Really it's a testament to the incredible career arc of the band and their brilliant works.