Mighty Brights do give the nicest light, from all I've tried too.

(A variety of clip-ons and the Amazon cover w/light.
I haven't tried the Sony cover with light, but might - can the cover fold all the way back and stay there...?)
I haven't tried the dual arm model Mighty Bright.
My favorite, because it's the most lightweight and uses 1 AAA battery, is the
Mighty Bright TravelFlex.
It needs a case to clip onto, though.
If you want 2 levels and don't have a case or mind the weight of 2 AAs, the
XtraFlex2 is great.
If you want the lightest light to clip onto the Sony or Kindle, the
MiniFlex is nice. It gives the usual good light, and you can get an adapter so it uses power from the house outlet, but otherwise, it uses CR-2032s, which I hate messing with.