The situation isn't great for ebook stores. Five of the six largest US publishers are setting their own ebook prices and requiring stores to sell at that price. They all include DRM on all their ebooks at all sites, as do many other publishers. There are some recent DRM-free ebooks, for example: many romance titles are DRM-free, FictionWise sells DRM-free "multi-format" ebooks, and Baen Webscriptions science fiction is DRM-free.
If you want to keep the DRM intact, then B&N has the least restrictive version but it uses your credit card number as a password which some people don't like. There are no limits on the number of reading devices per ebook, and B&N has reader apps for popular smartphones as well as the two Nook device variants.
The best option, though, is to immediately strip the DRM. This is probably legal to do in the US, providing it is only for personal use of ebooks you bought. It is easy to do with B&N DRM, and is equally easy to do for Adobe ePub DRM and Adobe ePub ebooks are available at many stores.
Once DRM-free the most open device is a Hanlin V3 (EZ Reader Basic) with the OpenInkPot open source firmware. This is what I use every day. The V3 is getting hard to find, because it is old technology. Other good alternatives include any of the PocketBooks, which support Adobe ePub but also have FBReader software both for DRM-free ePub and other formats (OpenInkPot also uses FBReader).
On Android, there are at least two good DRM-free reading apps, Aldiko and FBReaderJ, and several mainstrean ebook store apps. See
Android eBook reader clients. I am very interested in the Dell Streak, with a 5" screen, which will be sold in the US direct from Dell this summer (likely on AT&T, which isn't what you want). However, I'm not sure they will price it low enough to compete.