Alan Wake - horror game where your weapon is your flashlight. it's a long but great story about a writer who was enchanted to write a story that comes true
Amnesia/Amnesia 2 - great horror games if you haven't played them and like scary games
Binding of Isaac/Rebirth - RNG based adventure game where you start a fresh adventure every round and find powerups to crawl through dungeons
Dark Souls: Prepare to Die - a really tough adventure where you're going to die, a lot. if you're into that kind of thing, this series is great for you
Depth - a multiplayer game where two teams (formed by you and your friends, or npcs) take turns being either divers, or sharks. divers dive into the ocean looking for treasure, while sharks prowl around and kill them. really fun, unfortunately I personally don't really have anyone to play with
Escapists - casual strategy pixelated game where you're a prisoner attempting to break out of increasingly difficult prisons, fun and addictive, once you know what you're doing
Half-life/hl2/hl2e1/hl2e2 - technically horror survival but the story is great, I'd give them a try even if you don't like horror games, just turn up the brightness or something. you play as Gordon Freeman, an employee at Black Mesa when shit goes wrong and aliens are brought to the research facility, you are one of the only survivors and are tasked with ridding them and escaping the facility
Garry's Mod - a multiplayer sandbox game where you create and do what you want with friends, has modes for those a bit less create, such as prop hunt (hide and seek where you turn into props), trouble in terrorist town (a random person tries to kill the others), and a few others. one of the most worth it steam games you can get if you have friends who have it
Impossible game - casual platformer that's very challenging and is fun to tackle a few tries every now and then. you're a box and can only press space to jump over obstacles, music helps with timing
LA Noire - detective game where you solve crimes all over LA. its main feature is the facial software, where all faces are life like and you have to read them while investigating to determine if they're being truthful or not. tons of missions, i had a lot of fun with this game
LIMBO - simplistic 2D platformer with no dialogue where you're a little boy solving puzzles trying to find your sister, a few hours of gameplay, but you'll enjoy every minute of it
Lucius - not very well known game, you play as the son of the devil planning accidents to kill residents of a governmental residence. when i first got it, i beat it overnight, it was a lot of fun, but also really tedious as you have to do it "their" way (warning: one of the "accidents" involves a scene with brief nudity and sex, don't buy if you're under 18)
Magicite - a 2D survival game like minecraft where you gather resources, craft stuff, and kill stuff, but with permanent death. i don't know why it appeals to me, but it's pretty fun every now and then
Portal/Portal2 - if you haven't played them, go now. go buy them. you can thank me later.
Skyrim - one of the greatest solo adventure games of all time, everyone should play this atleast once if they like adventure games
Speedrunners - a multiplayer casual game where you run through a course with friends (or npcs) as fast as you can, using things like grappling hooks and power ups where you can grapple friends or throw rockets at them, really fun and somehow unknown game
Stanley Parable - a short game where you go through a really short, narrated story where you can choose to obey, or disobey the narrator. its only a few hours worth of gameplay, but when me and my so played it, it was probably the funnest few hours of gameplay we've had in a long time
Still Life - an investigative game where you're an FBI agent solving a serial killer case, where the serial killer is basing the killings off another killer a couple centuries ago. gameplay swaps between current time, and what the detectives were doing in the years ago murder case, overall a decent game, I enjoyed it. But stay away from still life 2, i raged a little more than I should have at how dumb the puzzles were
Super Hexagon - really fun music-based casual game, hard to describe so look it up if you're interested
Super Meat Boy - casual and fun, you play as a slippery piece of meat speedrunning through levels, handles incredibly well
Turbo dismount - casual game where you send vehicles towards obstacles or other vehicles to watch them crash in unique and fun ways, whether it's worth the money is up to you, but ive always found it fun every now and then
Walk in the Dark - 2D platformer with no dialogue where you play as a cat and its owner sidescrolling through levels, it's like limbo but a bit more creative
Walking Dead/400days/season2 - if you haven't played these, play them and prepare yourself for an emotional rollercoaster. seriously.
Don't think I missed any that I'd greatly recommend. I have a few others, but these are the main ones that i've played and really liked over the years.