I'm very fond of 'fontin' - which is free, so give it a try for nowt.... it's a serif font, but not too thin - unlike the sony font. It was a little 'light' in contrast on my 505 to a sans-serif Gill sans font, but looked that much better. Fontin looks excellent on my new blue 350! (did the Boroda hack without any issues).
Gill Sans is very good for a sans serif whether on LCD, ereader screen or printed out on paper - it's my default font in word on my Macbook (Gill Sans comes as standard with Macs...).
Tried Droid, but found it a bit ugly - the heights of the ascenders (?) seemed too short, or rather the bottom round bit of a 'b' (for example) seemed too big compared to the vertical line.
So far with a bit of preliminary fiddling with an html file in Word, seems like (for a 350) Fontin 10pt with line spacing at exactly 10.5pt (to open it up a bit) works nicely with Calibre conversion to epub. I leave Calibre on its defaults for font sizes etc, just change the margins down to 2pt all round.
Here's a pic of a page of fontin on my 350 as formatted above - the photo doesn't do the sony justice at all, unfortunately. It's surprisingly difficult to get a photo of these ereaders that really reflects how they appear in person....