Actually, none of them since the DRM is attached to the book, not the reader. Most readers will read EPUBs, which is the most popular non-Kindle format and MOBI (which is used by Kindle books in a modified form). The only way to be abe to read any vendors books on any reader is to strip DRM, which is tricky and has questionable legality (technically, it is illegal to strip DRM yet DRM violates fair use; most recent court decisions have been favorable for stripping DRM in certain situations, such as denial of use by a legitimate user). Even trickier is, often, you do not buy the e-book; you are buying a license to use the e-book. In its current state, the e-book industry is a mess and in turmoil due to DRM. To learn more about it, browse through these forums, especially the first two, rather than "listen" to me rant about it here.
A tablet comes as close as anything since Amazon (and B&N, I think) have apps you can install on them to allow reading their books. Cell phones with large screens can usually have apps installed on them, as well. The downside to that approach is the devices have battery life in hours instead of the days or weeks of e-ink readers. I use an e-book reader that uses a reflective LCD screen (less power hungry than backlit LCDs) and even it has a battery life of only 15 to 20 hours but it uses four AA batteries that I can easily change out in the field (all other e-book readers use non-changeable proprietary batteries).
I usually recommend Kindles and Kindle books to most people because it gives them the easiest way to get the best selection of books and are least likely to become obsolete (another potential evil of DRM). I personally do not like the Kindles (too bulky) and I have issues with DRM so, until I decide to strip DRM, I will not buy Kindle books myself. Most of my e-books are either free classics downloaded in EPUB format (if not EPUB, I use calibre to convert them to EPUBs) or image only PDFs of books I cut apart and scanned. PDF is a lousy e-book format but it is much faster than converting scans with OCR and correcting the OCR errors (the process takes even longer than reading the book) and I had almost two thousand books to scan (I'm still not finished). Not all e-book readers can render image PDFs into a readable form, especially the Kindles.