Minecraft
U.S. Unit Sales: 2.7 Million
Publisher: Mojang
Genre: Sandbox/Virtual Lego
Platforms: Pretty much anything with a screen by this point.
Minecraft has long been a time-sink and procrastination tool for both casual obsessives and people with far too much time on their hands, and it has capitalized further on its popularity by releasing on a wide range of console app stores.
