It kind of depends on what you like to play.
It's really a challenge to learn how to play but I personally really love Dwarf Fortress. http://www.bay12games.com/dwarves/ You can get a graphical pack so it's not so scary to start playing it.
It's a lot like minecraft, you can do anything! You build a fortress and manage dwarves. You can build farms, giant dining rooms, balistas, you can fight goblins, and make an entire industry to trade to other civilizations.
Here's the "lazy newb pack" for if you want to get started. It has tutorials and the graphical improvements that make the game playable (in my opinion)
I also really like bloodline champions. It is kind of in the same genre as League of Legends (DoTA) but it feels more like world of warcraft arena.