For me personally, at this moment, for reading on an e-ink display, the most readable font is Roboto. It was developed for (and paid for by) Google. It was specifically designed for reading on small displays with low resolution - a typical Android device.
You can download it legally and for free at
https://fonts.google.com/
The best feature of the font in my eyes is that it is extremely neutral. If you look at a screenful of text, nothing jumps out, nothing distracts. The font disappears and you absorb the text with as few distractions as possible. When reading a book I do not want to appreciate how well the font is made, how elegant it is. I have tried many, many fonts over the years and I have stuck with Roboto for the last 5 years or so. From time to time I have a look at what is new, what fonts are available, but I always very quickly return to Roboto.
But all that is just my perception of what is "the most readable font".
Go to the fonts.google.com site, download some fonts and start experimenting. It is fun, it is free.
For programming, for example I prefer to use appropriate font, such Source Code Pro from Adobe (that font creators at Adobe created for themselves when they write code).
A friend of mine can read with unbelievable speed. Like a thick novel under 2 hours with a good retention. Comparable retention to classmates who took weeks to read the book (that was decades ago while he was in school). He can read the entire line of text with one glance, so he just moves eyes in straight line from top to bottom of the page. There is a catch, of course ;-). It is difficult to concentrate like that for a long time and it is exhausting. So it is not suitable for leisure reading. There is another catch. The book has to be paper book with a proper typography in a serif font.
Some people say that the most readable fonts are those that were designed for highway signs, but I find those fonts distracting for long prose.
So ... how do you define the most readable font?
Is it the one that is least distracting?
Or is it the one that enables kids (very inexperienced readers) to read faster?
Or the one that is used for highway signs?
Or the one Adobe coders use for coding (that distinguishes between zero and capital O, between lowercase L and uppercase i and number one)?
Or the one that was developed after centuries of typography for books (typically serif fonts)?
Or the one that enables some people to read consistently 1000+ words per minute?
;-)
Go, look up a software for "Rapid Serial Visual Presentation" (there are several), select a font for displaying the text and crank the RSVP program up to maximum speed you can, while still comprehending text. Change font, repeat. See what font lets you read fastest.
For me, the maximum speed of 400wpm on Lexend page is too slow.