As long as its:
a) Not Times, Helvetica, or Arial (or one of the many copies of those)
b) Has a decent x-height
c) Is robust enough to be easily readable even on low-res devices
d) Has decent (at least full European) Unicode coverage
A matching serif and sans-serif is a huge plus for layout and design options.
I prefer seriffed faces for reading. Fontin looks nice and has a nice calligraphic feel to it.
PMN Caecilia used on the Kindle is nice (and has a beautiful italic) but misses out on decent Unicode coverage (only has West European).
Ascender's Droid family on Android devices looks pretty nice and is definately robust enough for low res screens.
SIL's Gentium is beautiful and has a huge character set, but is a bit too delicate for low res screens and doesn't have a matching sans. Then there's always the classics of Myriad Web and Minion Web from Adobe, designed for viewing on screen, paired serif and sans-serif.
