which font to use for consistent online experience?

Which is one (or more) font that is readily available for browsers on:

• Android
• iOS
• Windows
• Linux
• OSX

And that look very similar on every platform?

That's an issue Core Web Fonts are supposed to address but I think Microsoft has changed licensing terms so you can't quite count on same core web font being on Linux even thought the substitutes will be very, very close.

The best option is to embed any of the Google Web Fonts in your pages for everything looking identical. If that's not a option, Helvetica on a mac, Arial on a Win, and default sans-serif on Linux will be close enough that very few users could tell them apart.

I say go with Arial.

One of the easiest to read and is very consistent across all browsers.

You might be interested in this:

http://www.upsdell.com/BrowserNews/res_fonts.htm#a01