Does Black Mesa count?

I'm thinking Pokémon, exclusively due to Pokémon Uranium. Sure, they've done more than just that, but even just Uranium puts them in top place for me.

There are also amazing fangames as well to go with the franchise as well. Just in the last few years we have had demos/betas for games like Unbreakable Ties, Berilo, Flux, Eclesia, Azure, and Vanguard while also having pretty good complete games like Copper, Realidea System, Decay, Bizzare ( warning has some kinda cringy moments/things in it ), and Hidden Place.

Excluding romhacks, just the pokemon fangame community in general is probably one of the best for the amount pf things they have put out. Just within the last few years they have put out some amazing things like:

• Pokemon Unbreakable Ties ( very long beta with a real cool story attached )
• Pokemon Berilo ( much shorter demo but still shaping up to be real nice )
• Pokemon Flux ( shares a bit of the Pokemon Uranium pokedex while adding it's own fakemon and it's own battle gimmick )
• Pokemon Eclesia ( a demo with cool regional starters and a unique gimmick that will eventually happen that can make your run harder )
• Pokemon Vanguard ( has a long beta release that has a ton of regional pokemon and a few fakemon while also not being a collect 8 badges style pokemon game )
• Pokemon Conviction ( a demo pokemon game taking place in prison in an authoritarian region from what I can remember )

And that's just for uncompleted fangames. We have completed games like:

• Pokemon Copper ( typical pokemon game but has a lot of fakemon )
• Pokemon Realidea System ( has a cool out of battle gimmick and great art/story )
• Pokemon Opalo ( 2 separate stories split across 2 halves of a region based off of the US ( ugh, I know ) where the first half is more of a standard 8 gym adventure and the second half is no gyms but still crazy. Has plenty of regionals )
• Pokemon Decay ( has a rather interesting story with plenty of regionals and even some baby forms of non-regionals )
• Pokemon Bizzare ( loads of regionals, 3 fakemon starters, some pretty cringe moments, and a pretty good story overall )

There are plenty of others I could include, but these are all from the past maybe 3-4 years max. They are also not all the super well known games like Rejuvenation and Reborn since those already have gotten their fair share of attention.

Sonic is definitely a contender. they have enough new fangames out each year that rhey have a yearly festival, SAGE (Sonic Amateur Games Expo). some of them approach the quality of the classic games and have become classics in their own right, like Before the Sequel and After the Sequel. many of those devs have gone on to be successful indie gamedevs

Christian Whitehead is the developer of a fangame called Retro Sonic, and at one point pitched a port of Sonic CD to (then) modern platforms using the engine he built for it. SEGA accepted the pitch because the engine was so accurate (notable considering Sonic Team had recently failed at their own attempt and mismanaged another team's) and the port was so well received that he was hired again to port 1 and 2. those went so well that he was hired to do a new game and was able to hire a bunch of fangame scene alumni to create Sonic Mania, which was the best recieved official Sonic game in a long, long time. whether or not the Sonic scene has the best fangames, they most certainly had the most impactful on their respective official game series

Pokemon Unbound is far more than a mod to its base game (Fire Red), it’s basically its own GBA game. Not sure I’d say the Pokemon scene is the best but I wanted to call that romhack out for how good it is

AM2R (Another Metroid 2 Remake)

It's the story of Metroid 2, told in the style of Super Metroid. It adds a bunch of quality of life features and also adds new content that fits the world.

If you like 2D Metroid, you'll love this game.

::: spoiler Spoilers for my favorite part, just play do not read

The added Thermal Power Plant escape sequence was amazing.

You fight your way in, standard area. You fight a boss, who after being defeated begins a power plant overload.

Now you have a classic escape sequence. What you don't know is that the timer (overload meter) is not actually a timer, it moves up based on your progress. When you dug into the plant there were a number of slow moments, so now you have to slowly go through them in reverse. Finally you reach the top, the airlock. The slowest moving doors in the world. KABOOM!

You think you've failed. But Samus lives. You have 1 HP and have to limp two screens over to the healing room.

It's a cinematic moment in a video game. I'll never forget it.

:::

what's the thing with Undertale Yellow? i don't know much about fangames.

Zelda Classic (now ZQuest Classic) has made it pretty accessible to create a zelda fangame. Although the engine started as a basic Zelda 1 clone, it's matured quite a bit with a lot of new features and now even has its own scripting language. I've spent a lot of time playing some great creations and even made a few of my own.

Pokémon romhacks are on another level entirely.

I could not imagine being a Dev of Pokémon Emerald in 2003 and seeing Pokémon Perfect Emerald in 2023 seeing how utterly peak an already peak game could ever get.

Edit: I did not even see that OP specified no romhacks, whoops. Pokémon Hacks are so good they took away my ability to read.

I want to say the Bionicle fandom, but LEGO has made it clear to everyone that if your project looks too professional they'll shut you down.

Anyway, Bionicle fans have some incredible skillsets.

Luanti(was previously called minetest) has about 200 contributed game-modes in its mod-store, ContentDB.