Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
Here's a topic guaranteed to start controversy. Which Linux distribution is best? It all depends on your criteria for judging. Even then the topic is highly subjective. Here are a few nominees for "best distro" in specific categories.
Why does it matter which distros are most popular? They generally offer more resources: larger free software repositories, more tutorials, better doc, more support, and active forums.
You'd think there would be an objective way to identify the most popular distros. But reliable metrics don't exist for free software. Some measurements people use include SourceForge download statistics, stats on purchased OS discs, Distrowatch page hits, website tallies of visitor OS's, and Gartner Group and IDC estimates.