That really depends on where you are in the world. Worldwide 1366×768 is still the most common, however 1920×1080 is the only resolution that is actually increasing in take-up (presumably because this seems to be the standard widescreen for modern laptops).
The website StatCounter.com should help give you some further knowledge on this.
HOWEVER, bear in mind that just because certain resolutions are more common than others this it doesn’t actually take into account how wide the user has their browser window. Unfortunately, there are few known measurements for this statistic (Google analytics used to provide this, but have ceased supporting it).