After enabling the USE_ALT_FONTS "hack", I've tried many, many, MANY..... (let's just say, a huge amount of) fonts, but for some reason, I always come back to Caecilia 4.
It's not a beautiful font, but because of it's weight and plain representation, it's the easiest one to read. The facto that I have very poor eyesight (34% right, 7% left) is one to consider: it often makes that I choose fonts and themes on many devices for maximum readibility instead of beauty.
This enables me to read still read stuff while not setting a device to use a huge oversized font.
Many other fonts I've tried were too thick (e, a, and such get "filled up"), too thin, too spidery, too fancy, too small (needed to use size 5 or 6 to get a somewhat decent size, and it often didn't look good), just make me confuse letters; Baskerville is very thin, and it strains me to identify c or e at my reading speed, for example...
In the end, I always return to Caecilia 4.
It can also be that this is what I've gotten used to. IIRC, the KPW came with Caecilia 4 set as the default.
I've not been here for a long time. For people with a good memory: yes, I've upgraded my Touch to the Paperwhite as soon as I could import it to the Netherlands from Germany. My Touch is now owned by an aunt of mine. Best upgrade ever: the front light actually "finishes" the e-Reader for me. It's the one thing I've always wanted on the Touch
(By the way: why is it such a big deal for Amazon to let us add new fonts? My very first e-Reader, the Cybook Gen3, could do this out of the box, as far as I can remember.)