Best fonts for improving reading experience

Actually there are some pretty simple principles if your number one criteria is a high-readability font for a website.

1. Think mainstream. Thanks to @font-face you could choose from thousands and thousands of fonts – but less popular or newer fonts often have rendering issues between browsers and operating systems, sometimes even when they come from respectable foundaries. If readability is your priority, and you don’t want to do massive amounts of cross browser cross OS testing, you may want to play it safe and use a trusty, tried and tested font. The old web-safe fonts like Verdana and Georgia are safe bets since they’re so widely used, they will have been tested even in rarer browser/OS setups. Plus, familiarity aids readability.
2. High x-height, and wide. You don’t know what the quality or pixel density of your reader’s screen is, so, if you’re prioritising readability, make sure the important distinguishing features of the letters are extra clear. Fonts designed for web readability, like Verdana and Georgia, use as much as possible of their pixels for the x-height where the key details of a letter are, and use minimal pixels on ascenders or descenders where a one or two pixel bump is enough to easily tell a b from an o or a p.
3. Not too light. Light type looks great on print or high pixel density screens – but there’s a good reason why it was seldom used on screens until ios7 and was controversial even then: if the viewer isn’t viewing in optimal conditions, like a high quality screen in steady indoor lighting, it risks readability. Go for a font with a good solid regular weight – did I mention Verdana and Georgia?

There are plenty of fonts that meet these criteria other than Verdana and Georgia, of course, and these two are sometimes considered a bit boring because up until 2009 or so when @fontface became widely used they were been the go-to fonts for much of the web.

But you really can’t go wrong with them – they were designed for web readability, and every browser and operating system combination will have been tested to display them well.

Edit: When webtype sellers and foundaries do go to the trouble of testing for good readability, they’re likely to shout about it in their marketing. For example, the webtype “Reading Edge” series (hat tip to Chris Burton who mentioned this here). Still, do test widely.