Ohhhhh this thread idea is so mean but so good... It's so hard to answer a Top 10 without knowing it'll change if you ask me on a different day or without trying to come up with rules like "only one game from this series" to change my answers. Without ordering them (because I couldn't!), here goes...
Bloodstained: Ritual of the Night:
I love everything Bloodstained stands for. Growing up playing the metroidvania-style Castlevania games, they've become some of my favourites of all time and I adore the story of Bloodstained being made to really prove that people still want that kind of game and that it still has a place in people's hearts. It was a game I couldn't stop thinking about ever since I saw the announcement for it and was so eager to get my hands on it, so I quickly binged something like 100 hours of it right away. Ever since I've gone back to do things like a playthrough on the hardest difficulty where I beat the bosses without taking damage to earn medals, grinding everything to max level just for fun, and some dips into its randomizer mode.
Bravely Second/Final Fantasy V:
I love RPGs with the job system! In a top 10 list I'm very eager to throw in any Bravely game or FFV since it's what inspired those, but generally the reason I love them is mostly the same, it's for the job system. Going through the game and unlocking completely new ways of customizing every member of your team that can all intersect with each other means you have so many options to play an RPG how you want without things being too confusing or overwhelming. I never found myself completely lost on how these games work because they felt so easy to pick up and explained things well (which I feel RPGs can sometimes be terrible at, especially when you're new to a series) but felt so... open and customizable. I'd pick Bravely Second over Bravely Default II over Bravely Default just for their story and characters, but FFV feels like its own special thing that I can't leave it out.
Final Fantasy XIV: Shadowbringers:
FFXIV is the only MMO I've ever gotten into and ever since I got into it I've felt like I've never wanted to try any other MMO. I've fallen in love with the experience of playing it and almost having it like a second way of life, no matter whether it's things like raiding, crafting, socializing, committing to some kind of hard grind, I totally get how people feel about MMOs. I'm one of those people who thinks Shadowbringers has the best story of any expansion (though it can get pretty formulaic and boring sometimes!) and its story has some of those moments where you feel like you'd love to experience it for the first time again. Just entering the main city, the Crystarium, made me feel emotions no other game has.
Love Live! School Idol Festival ALL STARS:
This is... a weird pick? But probably an obvious one if people know me well enough. School Idol Festival All Stars (or SIFAS) is what really helped keep me a Love Live fan for a while as I was falling out with aspects of the franchise, and I've never played a game like it. Yes, it was a mobile gacha game that was a total grindfest, but it was a rhythm game with a bigger emphasis on using the cards you pulled to create a team that synergized together instead of just relying on the same skills a rhythm game would. Yes, I enjoy rhythm games and think I'm good at them, but SIFAS felt completely fresh. It was something that alienated fans of mobile rhythm games and felt incredibly niche, but if you
did enjoy it, there was nothing like SIFAS out there, and I'm so heartbroken it's gone.
The Legend of Zelda: Twilight Princess:
You could blame nostalgia for this if you wanted, but a Zelda game was either gonna be Twilight Princess or Wind Waker for me, they're both fantastic. But I have really strong positive memories of playing TP for the first time and things like its atmosphere and its music win out for me. I enjoy the more somber tone of it and there's just something about the whole trope of "the hero falling to darkness but he still powers through to be a beacon of hope to protect others" that just absolutely destroys me. I love TP's portrayal of Link the most and I adore the sort of melancholic feel the world of TP can have, the push and pull of sadness and hope the plot can have. I'm... waxing poetic about nonsense here but... it just
really vibes with me.
Castlevania: Aria of Sorrow/Castlevania: Portrait of Ruin:
I
love Castlevania and besides Bloodstained there's never been another Metroidvania that
really hits exactly like they do, I've tried and I might just be too nostalgia-blind. But I grew up with this series, will not shut up about how it's only the specific ones that came out between 1997-2008 and could explain the whole thing again... but I just
love Castlevania. I love exploring the gothic atmosphere, I love the enemies being things like undead monsters or creatures of myth, I love how good the spritework looks and how fluid they feel to play... pretty much any vania from this era feels like pure comfort for me and I could fall into it easily. I specifically mention Aria because I'll always recommend it as a fantastic starting point, it's easy to get into and its movement feels fantastic, and I mention Portrait because it's
usually the name I'll throw out as my favourite, I think it's got a lot of things that make it interesting.
Metroid Prime:
No surprise, given I like metroidvanias. Most of the reason I prefer Prime 1 is probably nostalgia, but I think it's incredibly impressive as a first attempt to bring Metroid to 3D. It didn't
need Prime 2 to fix the formula it tried to create, Prime 1 just perfectly took what made a game like Metroid so good and worked it into 3D. The nostalgic reasons I like it are just thinking it was really cool as the first of its kind, thinking things like Magmoor Caverns or the Plasma Beam or Thermal Visor were cool as a kid. Prime 2 is a fantastic game as well, I just can't tear myself away from that nostalgic love.
Persona 5 Royal:
I love P5R for a lot of reasons I've said I like other games here. Turn-based JRPG, a lot of interesting myth or lore or creatures as enemies, an RPG that's easy to understand... and I love Persona in general for that. I love Persona as a series I can really sit down and dedicate a lot of time to. I love how big the games feel because they go on for so long with the focus on the story, and I love the social aspects and how they intersect with things like the RPG aspects and the combat, things like social links or confidants are really nice. But the things that really make P5R stand out for me are specific to it. I love the themes of the story, things like standing up against oppression and false labels given to people by society, I love that the cast (and even confidants) are people who have been wronged by society or those in power and can find connections with those in common with them. I love how P5R specifically looks, it's always felt so flashy and cool and really exciting to play with every attack or menu. The gameplay additions to Royal like Showtimes or Persona Traits are great as well, but the best part is definitely that I think the third semester has the best villain in a Persona game, maybe even any RPG I've played. I love P5R.
Pokemon Platinum:
Pokemon Platinum is my favourite game of all time. I grew up playing it, I've played it too many times to count, I know so much about it that replaying it just feels like second nature to me. It was a game I played a lot to help me through problems in my childhood, it's something I've put an insane amount of time into doing things like the Battle Frontier or the black trainer card or getting every single item possible and finding ways to bypass events or transfer Pokemon from emulation to physical cartridges, there is no game that could come close to Platinum. I'm biased, I'm nostalgic, I don't care.
Pokemon SM/USUM:
I'm saying SM/USUM because they're mostly the same game and picking between any of the 4 of them is just personal preference. I don't want to yap about this one! I just think they're wonderful, I have great memories with them, and they came out at a good time for me that I spent really just engaging with Pokemon as a franchise and getting the most I could out of them.