And the chances are you've probably not even heard of it.
Early Access title Unturned is described by some as a cross between Minecraft and DayZ that sees players working together to survive an ongoing blocky zombie apocalypse.
An in-depth look at Steam user data by ArsTechnica has shown that little-known free-to-play indie game was the most owned game on Steam in 2014, with over 12m owners. A quarter of these owners are yet to play the title, however, although those that have clocked 9.1m hours between them throughout the year.
Only two of Steam's Top Ten best ‘sellers' are not free-to-play and only six of the Top 20.
When free-to-play titles are excluded the data looks very different. Goat Simulator tops the most players league table with an estimated 1.4m players. This is followed by Insurgency (1.2m), The Forest (1.1m) and Don't Starve Together (1m). Middle-earth: Shadow of Mordor, Banished, Dark Souls II and South Park: The Stick of Truth all reached 900k players.
When it comes to hours played per owner, however, Football Manager 2015 emerges as the outright winner with 152.5 hours. Next up is Dark Souls II (73.1 hours) followed by Panzer Corps (71.7 hours), Train Simulator 2015 (61.1 hours) and PES 2015 (60.4 hours).
Football manager 2015 has also had the most cumulative hours played by all owners at 130.4m. This is followed by Unturned (101.m), Dark Souls II (70.4m), Robocraft (56.6m) and Binding of Isaac: Rebirth (41.5m).