In the field, studies such as the one done by the Chitika ad network have found that Chrome OS now accounts for only 0.2 percent of Web traffic, while all other Linux distributions combined came to 1.9 percent. Put it all together, and Chrome OS by itself accounts for millions of users, but probably not 10 million users yet. After that, statistics break down completely. No one does a good job of tracking the "classic" Linux end-user operating systems.
DistroWatch, a site that tracks every Linux distribution like a hawk, comes right out and says it:
"The DistroWatch Page Hit Ranking statistics are a light-hearted way of measuring the popularity of Linux distributions and other free operating systems among the visitors of this website. They correlate neither to usage nor to quality and should not be used to measure the market share of distributions. They simply show the number of times a distribution page on DistroWatch.com was accessed each day, nothing more."
That said, their numbers are also the only ones we've got that have consistently tried to measure Linux desktop popularity. So, with their grain of salt in mind, here's what DistroWatch shows as the top three Linux desktops for not just the last year, but the last six months, three months and one month: Mint, Ubuntu, and Debian.
Yes, that's right, the days go by but this trio of Linux operating systems are always the top three. Based on that I think I can safely say they're the most popular classic desktop distributions. I, myself, happily use Mint every day for my desktop.
Beyond that, the other Linux distributions that are constantly fighting for a top spot are Mageia, Fedora, openSUSE, and Arch. If you're looking for a good desktop, I think any of these are worth looking at.