While Steam Spy is an "imperfect tool" by its own creator's admission, it's still a decent way to get broad strokes data on what's doing well on Steam. Creator Sergey Galyonkin recently pointed out the top six games released in 2015 on the platform thus far, and while the results aren't all that surprising, it's nice to have suspicions corroborated by stats.
GTA V, ARK: Survival Evolved, H1Z1, Cities: Skylines, Rocket League and Besieged have all sold over 1 million units since March 2015, the stats show. They're not the most newly owned games of the year – that distinction goes to Killing Floor 2, which has unreliable data thanks to a recent free weekend – but they're the ones that have sold most and have never been given away.
When you include games that have been given away, in addition to free-to-play games, the stats are very different. Evolve, H-Hour: World's Elite and Trove are among the most 'owned' games, though that doesn't necessarily mean a bunch of people play them: the mean playing time of H-Hour, for example, is just over four minutes, meaning a lot of people weren't drawn in by first impressions.
The Witcher 3 might be higher on the list if it weren't primarily a GOG title. Nonetheless it's sold more than 500,000 units, and so has Dying Light, Stranded Deep, Killing Floor 2, Total War: Attila and Pillars of Eternity.