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I think most of the fonts for ereaders now are pretty good overall; good enough that there is no longer a huge difference between them. However, older or classic fonts (like Georgia, Garamond, or Palatino) don't tend to display as nicely even if they work well in print.

Bookerly (used in Kindles) is a good upgrade over Caecilia for me. My issue with it is that the x-height (the height of lower case characters; specifically the 'x') is maybe a bit high (good for legibility, but I think it is a little too high to be balanced) and the glyphs are maybe a touch too regular (good for not having dark and light spots across a page, but a little robotic and boring).

Literata (used by Google's Play Books) is also quite readable, but I find it's italic very unappealing and it also has a very tall x-height.

Apple's Iowan is soft and comfortable. It has some of the variation in stroke width and letter shapes that I miss in Bookerly, and has "normal" looking italics unlike Literata.

The Chareink mentioned upthread is serviceable but too heavy to my eye. I find it tiring and prefer the original Charis SIL. Charis does have the advantage of including a very complete set of glyphs, so "foreign" languages will display properly. There seems to be a majority on Mobileread that prefers heavy and dark font rendering; I guess this suits them.

Note that there is a difference between intelligibility at a quick glance, and long term reading comfort.

I've chosen all serifed fonts here. Back when screens were in the 85-120 dpi range I often found sans serif fonts more legible, but now that dpi has reached 300dpi and up I have switched over to serifed fonts as being the more comfortable read.