Although it is hard believe that there could be a keyboard on an HP calculator that is more unreadable than HP 12CP, I nominate the HP-6S. This is the one with the Blue Case, not the HP-6S Solar which has a Silver Case. The key top is perfectly legible, but anything above the keys is almost completely unreadable in any light. I can see why the 6S is now discontinued. The main thing 6S Solar has going for it is the small size. Fits very nicely in a shirt pocket.
One other comment comparing the 6S and 6S Solar is that I would have thought they would use the same display -- but they don't, the display on 6S solar is bolder and more legible and the width of the zero character is significantly wider on 6S solar.
I know that these are the lowest of the low-end for HP, but I fear their construction gives me some idea of what to expect from the new and improved calculator line to be introduced this fall. I am hoping that the 33S will be a worthy successor to the 32Sii -- but I expect to be disappointed.