1. Ghost Trick: Phantom Detective. I know that there's a lot of stigma about games in which the story is the main focus nowadays, but GT is a shining example of how it's done right. That and it was written by based Takumi, who wrote the Ace Attorney series. In any case, Ghost Trick is full of life- the characters are instantly likeable and well-written, the art style and setting are gorgeous, the animation is flawless, the music is gorgeous and befitting of its neo-80's feel, and the gameplay is fun. It's one of my favorite games of all time, and I hope that more people play it. Cube picked it up and loved it, so the rest of you should too.
Similar/related choices: Ace Attorney, maybe TWD?
2. The Binding of Isaac. It's likely just because I'm a sucker for Judeo-Christian mythos and shit, but BoI is very possibly my favorite game ever. Extremely difficult at the start, but once you git gud it's smooth as butter. The fact that so much of the strategy relies on knowing how the game works and how to cut around it is loads of fun, but will sadly be absent for a while in Rebirth. The music is fantastic, the gameplay is fantastic, and it never gets old. 10/10 game, go buy it during the Steam sale.
Similar/related choices: FTL, Risk of Rain.
3. Half-Life, particularly Episode 2 and Opposing Force. I'm a fanboy, but Half-Life is fantastic, but especially Opposing Force and Episode 2. Opposing Force throws you loads of toys and tells you to go have fun while still being fairly difficult. Keeps that classic Half-Life feel but with more goofy fun. Episode 2 is great for the variety of levels, neat little outcrops, and variety of things to do.
Similar/related choices: Bioshock, Left 4 Dead.
4. Super Smash Bros., and I mean the entire series. There is truly no game as fun to play with others as Smash Bros. I've spent many, many hours playing Smash, and I don't regret a single one. Honestly, if you have even one friend who can hold a controller in their hands, grab a copy of any Smash and have a ball. Smash is also one of the finest hype generators I've enjoyed. The process of a new Smash is like slowly eating a chocolate bar.
Similar/related choices: Mario Kart, Skullgirls.
5. Mirror's Edge. As you guys may know, I have a boner for good atmosphere, and ME is the king of atmosphere, in my opinion. The story needs work, sure, and there could be less gunplay, but the world of Mirror's Edge is a blast to run through, creep around, or just sit and stare at. The music is a fantastic aide to the atmosphere and setting, reflecting perfectly the situations and places Faith is thrown into. Absolutely fantastic.
Similar/related choices: Portal(2), Jet Set Radio.
And other favorites that didn't make the cut:
Chibi-Robo
Elite Beat Agents
Hamtaro: Ham-Ham Heartbreak
Viewtiful Joe
Punch-Out!! Wii
Braid
Faerie Solitare
Onimusha
Pokemon
Limbo
The Stanley Parable
VVVVVV
Super Mario Sunshine, World, Bros, Galaxies 1 & 2, 64, and whatever the fuck.