As the new year approaches, Steam Spy creator Sergey Galyonkin has published a preliminary report suggesting that, with ~3.6 million copies activated, Rockstar's Grand Theft Auto V is the best-selling Steam game released in 2015, followed by Fallout 4 (~2.5 million copies) and Ark: Survival Evolved (~2.1 million copies activated.)
It's an interesting morsel of Steam market data (reproduced below) that game developers may appreciate, though it comes with a few important caveats that you should keep in mind.
Most importantly, Steam Spy is a work in progress that extrapolates data from public Steam user profiles and thus is never 100 percent accurate. Plus, it only launched in April of this year and thus this (nearly) year-end report only encapsulates activations of games on Steam from April 1st through December 20th.
Finally, note that Valve kicked off its Steam Winter Sale about an hour ago, which will likely lead to a significant amount of new Steam game activations in the coming weeks.
Galyonkin has acknowledged this, and plans to publish an updated version of his inaugural year-end Steam Spy report sometime next month.