Convince others, convince me, convince... Yourself?
I don't really have one specific favourite, it's a long list. But for now I'll choose:

Dustforce, one of the best platformers you can find. One that plays better on a... Keyboard? Yes, it exists. Dustforce is focused on perfecting levels, trying to do better each time.

You're a janitor, you have a broom and you must clean. Clean everything, and clean it all without breaking your combo to get an S rank. Completing an easier level gets you a key for a harder tier level, and boy do they get difficult. They don't even get annoying, due to the amazing Soundtrack by Lifeformed.

Highly recommend it.

Streets of Rage Remake V5.1

Free fangame, 11 years in work, bringing together all content from the classic, beloved series. Created from zero, using no (copyrighted) assets at all, even enhancing playable characters and stages with movesets and functionality correspondingly, much beyond their original capabilities. Abundancy of gameplay options and settings, enough to satisfy even the most hardcore fan; remixed soundtrack, 16 bit alternative one, even the possibility to manually set a soundtrack yourself! Ash, Roo, Shiva can be unlocked, even the final boss can be utilized; two more additional characters have been created, too! Comes with an editor and in online communities, lots of mods can be found; complete secondary games in and by themselves!

Pokemon Reborn

Take a series for kids. Add a dark story, in a darker setting, really challenging non kid-friendly gameplay (which requires thinking, planning, training), utilities like speed up button visible EV/IV and there you have it... A cult-classic, unknown-gem, silent-hit! Online mode present, too. And all little buggers, 807 of them can be found, inside one (1!) game, not scattered across a bunch of series... Episode 16 was great, but it had only up to and 6th gen. The current one has AI issues (to be addressed for the final episode, the upcoming one) and few movepool bugs (some critters cannot learn moves they normally should, or their move roster is partially wrong), but it contains up to and 7th gen, plus it features most iconic game features of the series as a whole, like Mega Evolve and Z-moves, as well as both normal and regional variant forms (alolan). Won't implement 8th gen, but hey, a similar fangame inspired by it, is going to; its sister, companion fangame, Pokemon Rejuvenation!

Assuming this has co-op, can you turn off friendly fire?

It HAS co-op, at least hotseat. But in a certain forum, there is a guide, to manually activate/set-up online co-op also, even!

There is an option for this functionality, of course (if you tamper with it, you cannot hit the other player; this deactivates automatically, if you take the SOR1 route and duel, accepting the offer of Mr.X, for the evil ending)!

Also, except for player co-op, there is even AI co-op, which it allows further options, around certain concepts; aggressive, defensive, item-collecting and much, much more!

This is the ULTIMATE collection! The only problem, is that officially, it has been ceased and decisted... But you CAN find it, together with community, which miraculously, is active to this day. Can't say no more.

Secret of Mana

Of the many golden children Squaresoft put out on the SNES, most of them feel rather aged in one circumstance or another. I wouldn't call Secret of Mana ageless, but I'd say compared to Final Fantasy IV, it's still got charms for the age.

It's an up to three player action RPG, with 8 directional smooth movement across a gorgeous world of very varying vistas, including forests, deserts, castles and more.

The story is a simple, "boy finds sword, is destined to save world" affair, but the way it unfolds between the actual concept of Mana is where the meat is.

Mechanically, you have 3 characters of whom you can either select between at will (as long as they aren't dead or preoccupied) or can be controlled by others with drop in multiplayer. There's multiple weapons to choose from for a variety of playstyles, and an equal amount of magic elements.

The score is gorgeous and perhaps what I would call Mana's strongest point, being composed by a legend who still works to this day.

Thief 2 The Metal Age

The immersion of sneaking through a steampunk world as a master thief stealing everything that isn't bolted down. You have a little of the mundane and a little of the macabre in the game. It's fantasy meats steampunk. It's about using stealth to hide in the shadows and in the words of Monty Python: "how not to be seen".

Unreal

No, not Unreal Tournament, that super fast multiplayer FPS you always hear about that you have unfortunately never gotten the chance to play during its heyday since you're too young...

No, this is UNREAL, and guess what? The quality of this game is unreal considering its age! Many uncultured normies (I am kidding) might say Half-Life is the greatest FPS game ever made (and that's fine, but I will act like that's a dumb opinion). Hah! Half-Life is like a smaller, more condensed version of Unreal. It's also trying too hard with the story, even having you meet and escort characters. What is this, a movie?! Get real.

Nah bro, if you like REAL video games, try UNREAL. Fast-paced action, some of the sickest weapons selections in any FPS, banging tunes and rad-looking alien foes, all set on the most beautiful alien planet in video games up to that point. Do you like adventure? Well, Unreal is one BIG adventure (especially for its time). Do you think most FPS nowadays are made for little kids that can't aim without their precious auto aim to compensate for their preference of analog sticks? Bro this game doesn't even have controller support, gtfo console kiddies (do not gtfo if you like playing FPS with a controller, that's totally fine!)

So if its so great, why did people forget about it?

Because it never came out on console, so anyone but the realest hardcore PC gamers would know about it. This game was so over the top badass that the console port got CANCELLED! You are a REAL PC gamer though, right? Then you should have played UNREAL and its sick expansion pack yesterday!

Zanzarah: The Hidden Portal

Think Pokémon, if it was made for PC, nearly 15 years later. You're a young London girl who gets transported to a magical realm of fairies. You soon get your first fairy and need to use that to fight other wild fairies of various elements/strengths/weaknesses in 1st-person arenas, which you then attempt to catch with balls, and use those to fight other fairies and so on - complete with leveling, evolutions, and whatnot.

There's a storyline, something about saving the fairy world, but I can't remember that very well at the moment.

An expansion was in production, but was never released.

Bit annoying that it isn't on GOG, I'd love to have an offline installer. You can get it at Steam though.

Actually, I'm not entirely sure it's my favourite (I'm not sure what my favourite game is), but it's good enough.

Will install on this computer too right away. Brought my savegame back home on a USB stick. Just a bit stuck in an underwater labyrinth :-/ and keep getting lost.

Hollw Knight
This game is a metroidvania style game with a desolate, desparate vibe. You are a nameless bug that arrives to a world that has been decimated by a force that has sapped all intelligence from them, turning them into mindless drones who will try to destroy you. Your quest? To explore Hallownest, the kingdom of insects. Along the way, you learn about others who have resisted this mysterious corruption, and find your true purpose, to end the corruption.

This game does not hold your hand. Death means losing your money until you find your void soul and kill it to regain your essence, There are MANY secrets, some of which you will absolutely miss without any help to find them all.

You will face deadly enemies, creative and lethal bosses, and along the way, you'll feel like quitting. But the gamer in you will keep pushing forward. It is not the hardest game I have finished. But it is also FAR from the easiest. Dangerous enemies and some pixel precise platforming will get you the best results in this game.

best write up ever! thanks to you i looked up the game and it looks marv, found out i had it in my library forgotten and never played, downloading it now.
my turn...

Do you crave a world of rampaging zombie hoards that surround and catch you by your entrails? Do you enjoy crappy graphics that somehow are more indepth than AAA titles? Do you enjoy building bases of such magnitude that the world shakes? then by golly I have a game for you brave soldier.. Project Zomboid. In perpetual beta cuz they are that laid back slacker cool. Amazing soundscape, long stretches of silence broken only by the moans of the undead or far off screams of the dwindling survivors. Lonesome music accompanies you as you search supplies against a timer of power failure, rot, other scavengers. Better hurry... winter is coming! what will you do for heat? will you freeze or starve? For now you better find some shelter, take the risk of noise and barricade yourself in because night is coming. There is nothing worse that pitch black that chokes while you can hear the shuffling moans of the dead coming for you.

in all seriousness i have yet to find another builder game that allows so much creativity in base creation. Do you want to make a sky base with limited access to upper levels? Would you rather customize an existing building or even city block. How bout tackling that infested Mall for shits and giggles. There are skill trees and starter classes to choose from. All the while the hordes are building in strength, if you dont take care of them now they will juggernaut over any and all defenses you made. There are mods galore and game modes you can select for hardness, survival, co op, scenarios as well. You dont have to build, you can run and gun till they catch you.

Where will you make your last stand?
https://www.gog.com/game/project_zomboid

https://www.youtube.com/watch?v=sLn9ho5D52Q
https://www.youtube.com/watch?v=OUCVDQvrpR4

This is the only correct answer. Awesome game.