I tried a few and found that BN eReader is a pretty good one, I like the way it can change the background in many different ways, like night mode, or customize the page colors etc., but it can be buggy at times.
BN ebook store features a huge book library, well, you know who they are. Also there's a lot of free titles too. Most of their books have the region restriction, but you can use VPN to access the store while purchasing, as well as providing a billing address within the US, since they don't really verify your credit card billing address.
By far BN provides many versions of their ereader for different platforms, such as PC, Mac, iPhone, iPad and Blackberry(I suppose). After purchasing once, you can sync your books with all your devices, and you can always download them anywhere you wish. (no limitation to any regions)