I will back this up ... I've been trying to complete my AMF run, and the 1/4 is a beeyotch. Only seen one on ebay, and I backed down so Win could snag it (he would have blown my bid out of the water anyways!). That copy was probably a fair, with brittle pages. It will likely be the last AMF I need... down to about five issues.
The AMF 2/1 is also tough, but not as impossible in my mind.
I can't chime in on any Centaurs beyond AMF, Funny Pages (hero/serious covers only), Keen Detective Funnies or Amazing Man. I've kept my focus on the hero/serious issues of those books. If I went for the "funny" covers, I'd be broke!
Toughest Funny Pages I own must be the December 1939 issue (vol3/#10) , with the Burgos cover. Meaning to get this pressed out some day... its got great pages...
I know the KDF 20 is not a super-rare book, but it does seem to be rare in high grade. Funny how some issues are like that... unless, there are minty raw copies tucked away in private collections we don't know about.