What's your favorite Pokémon Game?

Crystal is very special to me. I always felt like I was "wrong" as a child for not liking "girl things." It was the first game I ever played that offered a female protagonist. That was something that told me that I was okay the way I am. I know it's kind of silly, but it really helped me become comfortable with myself.

Emerald it gave me the first taste of the battle tower and I’ve been hooked since

In no particular order: Yellow, Crystal, emerald, platinum (oh shit, of course. It's all the merger titles)

My favourite will always be Red because of nostalgia. It doesn't really hold up these days but being a kid during peak Pokémon mania where everyone were whipping out their Gameboys during recess and the dude who got a Mew from some marketing event was king of the schoolyard was just the source of so many memories. Most of them related to longing for an innocence lost, I'm sure. But still.
Realistically though as a game I think HeartGold/SoulSilver was the peak of the series. Lovingly recapturing the essence of the first two generations, adding a ton of content on top and even shipping with the freaking Pokewalker! It's got everything that was good about G/S/C in a modern package, what more can you want? Going back to Kanto after beating the Pokémon League is still the coolest thing ever. But maybe I'm swayed by nostalgia again...
- I remember playing Blue so much as a kid, I would hear the MIDI based music whenever I closed my eyes.
  So glad we finally have the room on the carts for different music for each region/dungeon/route, but there something quaint about only having a half-dozen musical bits per game.

I played Gen 1 as a kid and didn't play Pokemon again til Black and White. It was an amazing leap to see the first and last pixel animated mainline Pokemon games, and both had their own unique set of 150 Pokemon. It's like a showcase of how much the series had grown.
Incidentally, it was the last mainline Pokemon game I actually enjoyed.

Until Arceus came along and finally did what Pokemon should have been doing a long time ago, it was Pokemon Snap. Arceus is great, but it's also the bare minimum of what I think the series should be. I would hope that Palworld has set a fire under Nintendo and GameFreak's ass to get them to make something that actually brings the series forward instead of keeping it mired in game design from a bygone era.

Pokémon Mystery Dungeon: Explorers of Sky
In terms of mainline games though, I'd probably have to go with Blue version, since it's the one I've spent the most time with over the years

Emerald holds a special place in my heart. Omega Ruby got me back into Pokemon and to this day is my favorite. I'm currently playing through every generation, but no other region hits harder that Hoenn

Ruby and Sapphire, 100% because of nostalgia as I haven't played either in 20+ years - those were the last games my grandpa gifted me so they hold tremendous sentimental value.
Gold and Silver would be close seconds, since my sister and I played those games when growing up and I have great memories from back then.

I think Gold was amazing back when I first played. I liked the new Pokemon designs and when I found out you can go to Kanto, my mind was blown. I did not expect so much playtime after the Elite 4. Everything in that game was an improvement to the first gen. It still had some jank, but at that time, I think it was the perfect game they could make.
But there is one thing where I think the GBA games stood out: graphics. I still love it, it's extremely timeless and just beautiful. That is still how Pokemon games should look like today. I prefer Leaf Green over Emerald because it doesn't have way too many water routes like Hoenn and as Gold and Silver, it had some nice unexpected post Elite 4 content.
Of course I'm totally biased because those were my childhood games. I tried Soul Silver maaaaany years later and got Shield for the switch, but they could not keep up with the GBA games. Maybe I will give Gen 4 and 5 a go in the future though.

Does Pokemon Mystery Dungeon count? I don't know which one exactly, but it was one for the GB Advance.
- Sure does. Any pokemon game, really. Did you play Team blue or red?
  
  I just looked it up and it says the only game that came out for the Advance was Team Red, so Team Red. Although I don't remember anyrhing specific (it's been more than 15 years after all), the whole game felt very cozy. I do remember playing as Cyndaquil however :) and that it was way harder that the mainline Pokemon games.

For Nostalgia? Red/Blue and Gold/Silver.
HG/SS and B/W are up there too for different reasons.
And funnily enough, I actually enjoyed S/V quite a bit. Aside from the lull between gens 6 through 8ish, there's a lot to like throughout the series imo. A lot to dislike too, but still a lot to like.

I have a soft spot for Crystal, in terms of the official series.
If we include rom hacks, Crystal Clear is great, and I'm really enjoying Pokemon Emerald Rogue these days.

I would like to say leaf green just because I played it later on and the little clubhouse you could make was cool, I recently started trying to replay one of each generation like yellow, crystal, and now emerald but man I just want to power through them but you have to grind to level up and i get sleepy

For me i would say silver for the nostalgia factor, Also silver (and gold) were compatible with the original black/white gb so Even more nostagic than Crystal

I've always enjoyed Firered and Sapphire and recently found Polished Crystal which is fantastic

Um.. one of the later ones for the 3DS. But I think these new ones coming out soon will be super awesome. I don't play many Pokemon games though.
I've played Blue, emerald, black? For 3ds? And that's it.

I only really played red and blue myself, but we did play the Green rom back before the English version came out. I think we stumbled out way along to the first gym. That’s probably my go to, since back in the day we completely exhausted ourselves playing it.
I watched my partner play Sword. It looked pretty good. She’s playing it again now with our daughter and it seems fun.
I started playing Soul Silver with my daughter on a DS emulator, and it seems good too.
I had Diamond on my DS years ago, but I only remember playing it for about an hour.

Fire red will always be my number one as it was my first game.
Otherwise it's a hard choice between Sapphire and Heartgold. Both had great regions and lots to do.
My most played would be Diamond and it was good, but sinnoh was never my favorite region.