Feature creep is already making this sound impossible in practice. My shortlist would be: FBReader, 10+ hour battery life, one popular DRM format (MobiPocket would be my 1st choice because I think Amazon wins in the end).
FBReader gives us most non-DRM formats, except PDF. If a (text plus images) file format isn't supported now it can be. Similarly, I suggest improving FBReader rather than re-inventing the wheel (dictionary, annotations, read aloud, etcetera). If you have Linux on a PC, download FBReader and try it out. It is Linux only, but see
When a win32 version? for an example of running FBReader under VMware Player.