So I never played any pokemon game. I was too old when it came out, game seems too childish, and I was never a big fan of handhelds.

I have been playing a lot recently on my cell phone due to life/work routine and emulation works pretty well for me. Also JRPGs are great for touch screens. So I thought I should try one pokemon game just to see what is all about. I know nothing about pokemon, just know the name of maybe 3 of them. I know its a jrpg style game and it is turn based. I dont even know if the games have a story.

So I want to play one. If I had to play only ONE pokemon game in my life, which one should it be? If possible a Gameboy Advance or a DS one, as 3DS might be annoying to emulate sometimes. Gamecube is also aceptable.

Pokemon fans, please help me catch em all.

Hard to say. Old ones are more challenging, but have aged visually poorly and lack many QOL features that the newer ones have. They feel like a chore to play because of that. Newer ones arent pretty to look at either, and they are more aimed at a very young audience, but mostly - are horribly easy. Theres zero challenge to them. My personal experience is that I lost interest very quickly because of that, there is just no fun in an RPG game that doesnt challenge you at all. 

The last one I enjoyed quite a lot was XY, but mostly because it was very oriented to the meta multiplayer community, without that its probably not worth it at this point. Legends Arceus was fun, so I guess that'd be my pick? Or maybe wait for the Legends Z-A to drop, see if it has good reviews

Blue

Pokémon Crystal, or Soul Silver/Heart Gold if going back to the Gameboy Color is too much for you.

Gen 2 is the best.

Silver

I think Black/White and to a lesser extend Sun/Moon are the perfect balance between being great Pokémon games and having quality of life improvements.

HGSS, Emerald and Platinum are amazing, but some stuff can be a little bit outdated and clunky.

HeartGold/SoulSilver is a good mix of old and new-ish.

Gold/Silver/Crystal, they removed some of the imbalance of the first gen, while still keeping the beautiful simplicity.

2D wise I would recommend playing Heart Gold/Soul Silver for DS or Pokemon Sword/Shield on Switch 1.

If you’re looking for the opinion of someone who is not a fan and doesn’t feel the series is for him, but has played several...

I’ve played 5 Pokemon games: Pokemon Snap, Pokemon White, Pokemon Y, Pokemon Go, and Let’s Go Pikachu. Pokemon Snap was a lot of fun. It features a character going around on-rails with a gun and shooting many Pokemon, collecting and grading those pictures you shoot. Pokemon Go is (for all intents and purposes) is over now, but I don’t think I need to explain. It was by far the biggest Pokemon game, and culturally significant.

Choosing between the other three is (mostly) irrelevant, as they’re all similar feeling games, they have almost the same story. They’re kind of like EA Sports games, where if you’ve played one, you’ve played them all, and the advancements don’t feel significant. All of them feature battles, turn based, but Let’s Go Pikachu has done away with random encounters, so encounters are all by enemies you see on the screen. Pokemon collection also differs. White and Y use the same system as Final Fantasy 8’s monster carding system, where you weaken them and then throw a ball, instead of using the card capture skill. Let’s Go uses a more active Pokeball throwing mini game. Collection is the main drive of the game, but if you’re anything like me, you’ll be tired of it after 30-40. If you care about iconic Pokemon, Let’s Go has by FAR the better Pokemon, White and Y had multiple times more Pokemon altogether, but I didn’t feel that I came across more. 150 or 700, it doesn’t really differ in my experience, just dilution of the ones you want. It’s a perfect example of how extra content doesn’t necessarily improve the game, and in some ways it takes away from it → Chrono Trigger DS, Sword of Mana, and Final Fantasy 6 Advance, I’m looking at you. But, off that tangent. Of these three I found Let's Go Pikachu to be significantly more fun and satisfying than the other two, but it's also a newer game; Pokemon Go was more of a social experience; and Pokemon Snap was probably the most fun, but it runs out of steam quickly.