I still love
Linux Libertine and Linux Biolinum, mainly because I use these when producing PDF/real books. Then again, I have a 7" 300ppi reader, and these might not be the perfect fonts for any eReader.
When I want
real good Unicode coverage, I use a combination of
Kurinto Text (serif), Kurinto Sans (sans) and Kurinto Mono Narrow (monospace). These are huge fonts, due to their number of Unicode glyphs, but there exist "tuned-down" smaller versions (labelled "core"). Kurinto fonts are carefully tuned for size, so you can mix-and-match different languages and writing systems easily.
The attached screenshot from my Tolino Vision 5 shows text in
Kurinto Text (note sub- and superscripts, greek delta and the asterism), and a headline in
Olde, a blackletter font from the Kurinto project. Disregard bad formatting—this was just a test for embedding font subsets into an EPUB, to keep the file size manageable.