I think I've played too many games to pick favorites... Well, I could still try.
5. Mafia II - Oh, boy. This game is like a mafia GTA type game, and I love it. It's got an interesting story, and all the character's are very developed and compliment each other well. And also Vito's face.
4. To the Moon - Someone already said this one, but holy crap, I love this game. It's the best RPG Maker game (I think at least) out there, and it honestly makes me cry ever damn time. I even cried when I watched Markiplier play it. (Might I also add, one of the only things ever to make me cry.)
3. Banished - Something about this game is so drawing and addicting to me. You simply build a small village with a handful of banished (heh) villagers, and you make do. You collect resources and tell your villagers what to do to benefit the town and themselves. They all have to work together to survive and ensure that the next generation lives.
2. Portal 2 - Portal 2 is here because I was so freakin' obsessed with this game. I would say that it is the most fun I've had in a game to date. It's got a fantastic story, and GlaDOS and Wheatley are up there in my favorite character list. Speaking of which (if not done already), someone please make that list.
1. Mass Effect Series - I couldn't pinpoint my favorite game, so I just did the whole series. Holy hell, I love these games! I haven't gotten the chance to play Mass Effect 3 yet (which I hear is the best in the series), but from playing both 1 and 2, I can say that it is the best few freaking games I've played to date. The story is ungodly good (in my opinion, at least), the gameplay is fun as hell, but most importantly, the character are the best bunch of people I've ever experienced! If you haven't played these games, do yourself a favor and play the trilogy.
Honorable mentions: Smash Brothers (best fighter ever), Fallout 3 & New Vegas (I'm so sorry Banished took your place), Team Fortress 2 (900+ hours, oh god), Don't Starve/ Don't Starve Together, Garry's Mod (hell of a time), This War of Mine, The Sims 3 (played this for about 2 months straight), Dust: An Elysian Tail (lot's of platforming fun!), TES V: Skyrim (of course), Hearthstone, Paper's Please, All the LEGO games (don't judge, I loved those as a kid), Pokemon 3rd Gen. (screw the haters), LOZ: Wind Waker (Toon Link ftw), Soul Caliber 4 & 5, Half-Life 2 (and the rest of them), and finally, Left 4 Dead 2 (KIDDIELAND!)
FORGOT ABOUT ALL THE TELLTALE POINT-AND-CLICK GAMES, OH LORD (They are fantastic. I'd say Tales of Borderlands would take number 5)
Thanks for partaking in my incredibly long list, I just thought I'd post it