Probably going to cheat a little bit here, but going to go with some common recommendations but a few niche options on top of that depending on what you may be looking for.
Halo: Master Chief Collection - Halo 1-4 and ODST in one convenient package. Only missing Reach.
Life Is Strange - Probably my Game of the Year for 2015. If you enjoy story driven games, can’t do much better. Scenes resonate with me to this day and still get emotional over certain parts. Five total episodes at about 2-4 hours each.
Overwatch - Team based shooter with multiple ways to play. Also different character styles means there’s something for everyone. ‘Best’ part is, unlocks are purely cosmetic so you never have to worry about someone unlocking better equipment than you.
Rayman Legends - Possibly one of the best platformers this generation. Music levels are a blast and includes multiple levels from Rayman Origins. Almost two games in one with the amount of levels. If you beat the single player campaign and want more action, daily and weekly challenges await.
Rare Replay - 30 Games for the price of one. A few clunkers (mainly the ZX Spectrum games), but lots of hits. Only hit it’s missing is Goldeneye, but it has Perfect Dark which is arguably better.
Above list is a general, please everybody type of list. The below are recommended either because of their price of entry or part of a niche genre.
Killer Instinct - If you enjoy 2D fighters, this game is solid. One character rotates weekly for free which gives you a small taste to see if you enjoy the game or not. 24 total characters in all.
Magic Duels - Another freebie with micro transactions. If you’ve ever wanted to try Magic The Gathering, this series is a great learning tool (learned myself from the 2011 game). There are deckbuilding restrictions which may be a turn off for veterans of Magic, but for someone new, it’s a solid start. Be forewarned that addiction may cost you more money than drugs however.
Raiden V - Not many traditional shmups exist on the marketplace, but the Raiden series hits it out of the park with another great addition.
EA Access - Not so much a game as it is a service. Anytime a new game developed by EA is released, you get to play 10 hours for essentially free, but the main selling point is the vault. Generally within 6-12 months, games will hit the vault which means you can play them as long as you’re subscribed to access at no additional cost. While there are a number of sports games, there are others as well. Mass Effect 1-3, all of EA’s shooters (not counting Titanfall 2), and Mirror’s Edge 1&2 to just name a few. $30 a year is a steal, but if you’re not sure if you want to commit that at once, you can try the service for $5 a month as well.