From the default choices, I still like Caecilia Condensed the most. If they update the e-ink Kindles with Bookerly, however, I'd prefer it. It looks rather nice on my iPad.
For almost all of my reading, however, I sideload and embed a custom font. I use a fairly heavily poked at (at this point, I've made so many little tweaks, I'm not sure what all I've done to it) version of Edlund.
After all my early font experimentation, I've settled on it and have been using it for about 1.5 years
.