When considering a typeface, I would try to balance width (regular, narrow, condensed, or compressed) with size (rendering at 14px vs 18px+). Generally as typefaces become narrower it’s more difficult to quickly scan at smaller sizes, but it all depends on how well the typeface is drawn for screen, so digging in and testing is usually the best approach. Amount of personality (quirks/irregularity/things that stand out to a reader) is up to you, even if the recent trend has been toward ‘clean’ sans-serifs, that doesn’t necessarily equate to higher readability.
For the extended answer, data heavy doesn’t mean there aren’t breaking points where the amount of information is too dense for a user to consume in meaningful ways. I’m curious if data-heavy implies you're dealing with tables? There are a lot of different techniques for increasing capacity across different screen sizes.
Since you’re specifically addressing fonts (or more broadly typography), Erik van Blokland has some thoughts on the subject of responsive fonts Erik van Blokland – Responsive Fonts.
Also, many well established foundries today offer a version of a typeface optimized for use at small sizes, and these usually have wider proportions and lower contrast, if that's any indicator of good widths to use at smaller sizes: ‘ScreenSmart’/‘MicroPlus’/‘Reading Edge’
Fantastic reply, thank you for that.
Yes, data-heavy, in this case, implies I'm dealing with tables and forms.
Hope it helps :)
Source Sans is probably one of the best free options, and it's part of a collection of families that offers really wide language support (and growing it seems).
I didn't include any serif families, but that's another area of the current trend in web typography that might be worth trying to buck.
Also, newspapers have many of the same typographic requirements as designing for high-density data-heavy interfaces on screen: Designing News
Source Sans is relatively narrow, so you can fit lots, but still quite legible. Would be a good choica. Same thing could apply to Roboto.
I'd look at Work Sans; it has good contrast and works well at small sizes.
I'd suggest ones with a large x-height and very open counters, if you mean to display lots of dense text at a small size with maximum readability. Something like Tahoma or Verdana come to mind.
Typography legend Erik Spiekermann designed the typeface Axel for exactly that purpose: https://www.fontshop.com/families/axel
Open Sans is one of the most performant Web Fonts out there.
General purpose typeface Lato and for numbers or data included in tables Source Code Pro. Both are availables on Google Fonts for free
We use Roboto at AppSignal, because it's relatively narrow and still legible, with multiple weights available to pick our own regular / bold combinations.
My personal preference sometimes shifts to Source Sans, or not using a webfont at all for the sake of performance.
PS: It's a typeface, not a font: http://www.will-harris.com/font_vs_typeface.html
We use Roboto at Hightower as well, but I was curious to see what else is there.