I will second this.  Not because I play it, but because I have a friend who plays it and ignores all his non FF14 friends for all his FF14 friends.  He's addicted.

Counter Strike Global Offensive if your into FPS it's very addictive (edit just read one of your replies nvm).

Europa Universalis 4 if your into RTS (it's much more in depth then civ especially if you love micromanaging, but steeper learning curve)

Haha, came hear to say the same thing =P

For some different suggestions

Dungeons of Dredmor - Basically a super accessible Roge-like with graphics and stuff. Endless dungeons are a good time sink.

Avernum: Escape From the Pit - a true old school cRPG but it's not too hard to get into. Dozens of hours of content.

Mount & Blade - Combining an open world historical RPG with some lite battle strategy elements with some of the best first person melee combat around.

Path of Exile - A F2P (the good kind, the only gameplay relevant things you can buy are stash expansions) is basically a DIablo clone with some new and unique twists.

Civilization
Also if you like the idea of an alien vs. marines, asymmetric FPS/RTS hybrid check out Natural-Selection 2, because that's exactly what it is.
Here:
http://www.youtube.com/watch?v=AB-gmyOvUig
Keep in mind that the game is still continually updated.

More LoL, WoW, Path of Exile, FFXIV, and not game but lots of anime.

No one suggested Sins of a Solar Empire? Shame on you all.
You can waste days just on a single large, multi-system map with a ton of players, without a sleeping break.

After a few hundred hours spent playing that game since I bought it, I won't play it *ever* again, because it takes up to much time.

The "Humble Sid Meier Bundle"... includes Civ 3 - 5
https://www.humblebundle.com/

Civilization V with expansions Gods & Kings and Brave New World. On Humble Bundle you have to pay $15 to get all three. You can also get Civilization IV: Complete Edition for any price on Humble Bundle.

https://www.humblebundle.com/

eve online, biggest time sink ever.