I discovered today that my most played game on Steam is Astronner with about 500 hours, and my second most played is Surviving Mars with about 500 hours.
There are two games that I have spent a considerable amount of my life playing Planetside 2 and Path of Exile.
Planetside 2 released it was exactly the game that I was looking for. The experience of playing in an organised outfit was the best multiplayer gaming I've ever experienced.
Path of Exile is like a sledgehammer of a learning curve. Never had played an ARPG before this but once I got to maps my soul was taken indeed.
Before this left4dead 2 was the game that I had suck many hours into - what a game!