Uhhh, please no Helvetica.
I use Caecilia most of the time, just because that was the original and I got used to it.
However, with the original firmware my Kindle had shipped the font rendering was different, it was less sharp and the boundaries were less prominent (i.e., more comfortable to read). That's why I still desperately want to downgrade, but unfortunately it is not a possibility at the moment.
Anyways, thanks for the answers!