If you read my Day 3 post, you already know who tops the list with no contest, but I'll give you a few more just for good measure. Where available, I'll even list how many hours I've put in to these gems.
5. DCU Online: 48 Hours
DCU Online was a fascination for me for a long while. The character creator was fun to play with, as I tried to copy certain heroes or create my own. I finished only a couple of campaigns, but I spent a ton of time dashing, hopping, or flying around Metropolis and Gotham. Eventually, though, the repetitiveness got to me, and I haven't touched the game in months.
4. Sins of a Solar Empire: Rebellion: 48 Hours
This intense space-based real time strategy held me in even when I wasn't doing well, solely because of how great it looked. The camera could zoom out and watch the entire fight, or pull in close to watch a massive battle play out from the perspective of a fighter pilot. Single games can last hours or days, I still have ongoing games saved and ready to pick up at any time. All around it's just plain beautiful. I've never enjoyed watching myself get destroyed as much as I did in Sins.
3. Grand Theft Auto V: 110* Hours
Why the asterisk? Well, in the PC version, I've put in 56 hours. However, I previously owned the 360 version, which I played at least as much in, but without an official count, it's hard to say. But why play so much, and on two different devices? Three words: dynamic open world. No game has made NPCs as interactive and lifelike as GTA V, and each inch of the world had incredible detail. First person mode made it feel like a whole new game, and heists in Online essentially added a full second campaign to an already expansive game. If you're looking for pure value per dollar paid, GTA V has absolutely no competition.
2. Mass Effect Trilogy: 150 Hours
Once again, I'm grouping the three together, cause you really don't get the full experience of the games if you only play one. Mass Effect is my favorite game series of all time, with the best story and some of the best written characters in video games. If I could only ever play one game series, it would be this. Shepard and his/her crew are just about the most formidable team in gaming.
1. The Elder Scrolls V: Skyrim: 372 Hours
Mods, glorious mods. Though I put many hours in to the base game, Skyrim pulled in a couples hundred more from me purely due to its bountiful harvest of mods. I toss children in the air, blast giants and deer to outer space, and send armies of goats to attack villages. Of course, not all mods are that ridiculous. Some add spells, armor, and quests, a few even add entirely new areas like Elsweyr or the non-canon Wyrmstooth, a personal favorite. The game itself is fantastic for its world design, mechanics, and story, but its customizability truly shines through. The PC version is easily the preferred version of Skyrim for those reasons.