Platinum or Heart Gold if you want something to play on your phone, X/Y if you’re using a 3DS emulator on a computer, Sword/Shield if Switch emulator.

Explanation of my answer because I’m high and felt like writing it out. I started playing Pokémon with Ruby and have played nearly every game since then, the only ones I’ve missed are Black/White 2 (2012), and Ultra Sun/Moon (2017). I have heard good things about both of those though.

Starting from gen 3: Ruby and Sapphire are obsolete because of Emerald. Emerald is very good, but it predates the “Physical/Special Split.” Prior to Diamond and Pearl, whether a move uses physical or special attack was determined by the move’s type, which sucked because it meant moves like Fire Punch wouldn’t use physical attack. This changes the games fundamentally and is a huge improvement. I love Emerald, but when I go back and replay it I use romhacks to implement the split.

Gen 4: Diamond and Pearl are obsolete because of Platinum. Platinum is a finalized version of D/P, it’s got a great story, and is reasonably challenging (for a children’s game). Cynthia, the champion, is generally remembered as one of the best battles in all of Pokémon. Heart Gold and Soul Silver are remakes of Gen 2 and are fantastic, and like Gen 2 they’re longer due to the Kanto second half. Obviously you know the story for those.

Gen 5: I liked Black and White a lot, it has one of the most memorable villains in Pokémon. I never played Black/White 2, but I hear they’re even better. My main reasons for suggesting Platinum over these are the champion fight and my personal preference.

Gen 6: X and Y are the first games on the 3DS, and the first games to introduce the Fairy Type. From here on each game has a battle gimmick, in this case it’s Mega Evolution. Certain Pokémon can Mega Evolve in a battle. Boosting their stats, giving them new abilities and sometimes changing their type. Mega Evolution is generally remembered as the best of the battle gimmicks. To be honest I don’t remember the story to these ones much, but I don’t remember disliking it.

Gen 7: Sun and Moon were the first Pokémon games I disliked, and the only one I played at the time and didn’t finish. I found the characters annoying, thought it was too easy, and at the time didn’t enjoy the changes to the Gym system. Also possible part of it is just that I was a moody teenager at the time. Ultra Sun and Moon are apparently significantly better and are very popular, but I haven’t played them so I can’t say. I believe Gen 7 also has the Mega Evolution system.

Gen 8: Sword and Shield are… okay. The better of the main Switch games, but I can’t say they were my favorite. Not a good challenge, I found most of the characters kind of annoying, although a couple of the gym leaders are pretty good. The gyms are mostly giant stadiums instead of small buildings, which is fun. The gimmick in these ones is called Dynamax/Gigantamax, Pokémon get real big because they wanted to show off what the Switch could do. It’s… fine. Not as fun as Mega Evolution. Legends Arceus is also considered a mainline game but it’s veeeery different, I highly enjoyed it though.

Gen 9: Scarlet and Violet are bad. They’re a buggy mess, they run like trash, they’re ugly as sin, they’re easy, they’re boring, and the characters are almost all insufferable.