I cannot decide between my own versions of Veranda and DejaVu Serif.
Veranda is a bit more readable to me (strange, because I *hate* sans serif fonts on paper), but I like the "bookish" look of DejaVu Serif more. I'm switching every chapter or so
It could be that Verdana is so readable for me because it is by far my preferred font on the computer. I've been using it for 15 years or so, maybe longer. Since it was introduced by Windows; for me, that'd be 1996, with, Windows NT 4, I think.
And as much as I like Verdana on the Kindle, I despise Helvetica. I don't know why, but that font just seems "wrong" or something. I lose my line often, and it's just plain hard to read. Don't know why.