Pokémon (Gen 1). Both as a very good and unique game at the time and as an outstanding sales success that paved the way to making Pokémon the biggest entertainment brand in the world.

Resident Evil - launched a major franchise and popularised survival horror. To this day, I still think this game oozes a certain atmosphere that is near unmatched

Tomb Raider - launched a major franchise and is a fantastic game that creates an incredible sense of isolation, exploration and wonder.

Yoshi's Island - is this classed as debut game? It's considered part of the Mario canon, but also the first Yoshi game. If yes, then it's just a top notch platformer with great level design, music, graphics, and it just an all round fun experience.

For me the first Diablo takes the spot of best first game in a series.
It birthed the modern action RPG. To me it is the best in the series. The atmosphere of the game is unmatched. I can play and replay it endlessly.

This.  Beat me to it.  Exactly this.  It created an entire genre that so many are trying to replicate and Elden sold crazy well.  

Thus, in recent times the winner is Demon.  If we do historical, has to be Super Mario Bros, I mean it brought gaming back from the dead and created a massive industry.

Not only created a specific sub genre to action games, but also changed how other genres are designed to be similar to how Dark Souls tackle some things like healing, checkpoints, losing currency/progression after death, stamina-based deliberate and slow combat, etc

It's interesting how action games were reading to a completely different path on mid to late 2000s, and then Demon's Souls came and craved a new branch of games developed with Souls philosophy 

Fair point.  Silent Hill F, as a recent example, has a Souls like battle system, which is one of the reasons it was superb.  

I'd also give nods to:

Dead Space, for nailing its atmospheric depiction of sci-fi horror with its unique dismemberment-driven combat.

Halo: Combat Evolved, for revolutionizing the FPS genre to the point where it still feels relatively modern more than 20 years on.

Xenoblade Chronicles, for its unforgettable world, charming characters, and sublime soundtrack.

Awesome pick.

100% agreed on how its atmosphere is unmatched.

Also, Tristam theme... *chef's kiss*

It's not been mentioned yet so Portal. I like the 2nd game even more due to its spectacle and co-op mode but the 1st was genuinely a 10/10 execution of what it was going for that being a short puzzle game with a really cool and unique central mechanic combined with a compelling narrative and atmosphere. Everyone into video games who doesn't outright detest puzzle games should play at least the 1st one.

Dragon Quest, it invented JRPGs.