No matter what kind of reader you use, I suggest you to remove DRM anyway. You system may crash, your reader can break down, your next reader may support other formats only, and it is always easier to convert DRM-free ebooks.

I like Sony Reader. But it doesn't support mobipocket. You need to remove DRM protection and convert files to use it.

Amazon Kindle could be a better choice, since more ebooks are available for it (See how many ebooks are available in the Kindle Store). And Kindle Development Kit should make Kindle even better, since the should be more applications for Kindle.

Note that there are dozens of other ereaders on the market. If you live outside the U.S., you may find that some other reader works better for you (because of the localization, format support, warranty,...)