Google Chrome
Chess.com Browser Statistics!
If Opera had a better name, more people would use it.
/2c
Opera got its head head lopped off because one has to be a super-geek to get the most out of it. It is a great browser, and for coders and programmers Opera is fantastic, but the heavily hyped open-source Firefox which led to the massive development of custom extensions and themes blew Opera off the map and it will probably never come back to mainstream success.
Much faster.
http://lifehacker.com/5352195/browser-speed-tests-chrome-40-and-opera-10-take-on-all-challengers
Also check out this test, change the resolution to high and compares it with Firefox.
http://www.benjoffe.com/code/demos/canvascape/textures

Im suprised that many people still use IE and how little people use chrome, im using chrome right now, but I switch between that and firefox. Firefox is safer and has better addons, but chrome can be faster and a little more convenient. There are small things chrome does better than firefox that makes me wanna use it.

I'm guessing Chrome usage will spike real soon.
Firefox all the way!