Tough question but I would say.... Half Life 2, Portal, Gmod, Bioshock 1, Tomb Raider (has really no one mentioned this yet?) and Rust.
I would also say Arma 3 even though I haven't played it yet but from what I've seen it looks awesome.
The Meaning of Life, the Universe, and Everything.
I'm not entirely sure. But it would probably have to be Sonic Generations.
Despites it's short length, it's a pile of fun and has a ton of replay value. I got about 30 hours out of it just unlocking all of the achievements, and if you have the Steam version theres all of the mods for it aswell.
Hell yeah it's the best game. In my opinion every Elder Scrolls game is amazing, but the latest one is going to be the one I remember. If it wasn't that, there isn't much else apart from Star Wars Battlefront 2 (anyone else hyped for 3?), but there's also WoW to consider, not as best game, but it was definitely fun for me until Pandaria.