I've tested and used:
Hanlin (Papyre 5.1)
Onyx Boox 60
PocketBook 903 Pro
Kindle 3 (Keyboard)
Kindle 4 (NT)
Kindle 5 (Touch)
And winner is... Kindle 4 because buttons are soft, it can be hold with only one hand and can easily be pressed by side and by front when holding with two hands.
If you have opportunity, take one K4 and test it.
keys are sided at 45 degrees, that is you can "hold" with finger printer and doing a rotating pulsation with knuckle you get page done. But keys extend to both sides (front part and side device part). They makes a chamfer. I do not know how to explain in English, but you can see one hi-res image here: http://media.corporate-ir.net/media_...angle%201).jpg
Of course, to use with only one hand you need to have no cover.
Second one is Sony T1 because it can be handled with left hand and press second button without problem.
For the older ones, Boox 60 was a great option because you can pass pages with the big round joystick.