(Are we allowed to keep this thread going even when the user has been banned from PC forums?)
I find this question difficult to answer, and I need an answer because I want to introduce my nephew to the Pokemon games when he's old enough to read the dialogue around age 7 or 8. (The anime is different for obvious reasons, so I can show him that at age 3 or 4). My favorite games are Gen III, but the graphics will definitely be seen as dated by a lot of kids now. Those games don't tell you which moves are physical or special, and there's the lack of physical/special split, but I genuinely can't imagine a beginner as young as 7 caring about stats. Most of us picked Pokemon for looks or because of the anime at that age. (I don't know what age kids start questioning whether graphics are old or not. A friend of mine said his younger brother who was born the year I was got into Pokemon with Red AFTER my brother tried to show him the newer games like BW and XY.)
The BIG problem with Gen III, of course, is that you have to memorize a very intimidating chart to know which moves are effective, super effective, not very effective, or ineffective. It starts off logical that fire burns grass, water puts out fire, electricity zaps water, etc, but it becomes convoluted with stuff like "ice is frozen water, so water resists ice" and "grass absorbs water" or "Fairy-type beats Dragon-type because magic defeated the dragon in Sleeping Beauty" (or something like that).
So going backwards from the current games: Scarlet/Violet is a very weird and experimental game. There's hardly any trainers, and TPCi said you can challenge the gyms, titans, and the criminal organization when the devs clearly didn't intend for you to do that. Yes, my nephew can beat every Gym Leader first, but that'll make Team Star way too easy, and it's very difficult to explain to a seven-year-old, "Hey, I know you're excited, but you shouldn't challenge the ice-type leader with your fire-type starter first. Try the bug-type gym or grass-type gym first instead." This is one of my two main complaints about Violet; my other problem is I feel trainer battles are too spaced out, which is fine for one entry, obviously, but I hope they don't continue that in Gen X (that's Gen 10--confusing, I know).
I don't think I need to explain BDSP. Some people defend it to some extent, but I haven't seen anyone say they're great or even good Pokemon games.
Legends Arceus is way too experimental for a first Pokémon game. I haven't played it, but for some reason, your main objective is just completing the PokéDex, which is a bit reason I haven't played it.
Sword & Shield are probably the ones I'd like to show my nephew. It's a fun little adventure, but I get a bit fatigued by the 5th gym, and I haven't completed my most recent playthrough because of that (which is also why I haven't played Isle of Armour and Crown Tundra).
Let's Go Pikachu and Eevee is complicated cuz Pokemon player I've talked to IRL has recommended that first, but... it's boring, and I don't want pretend to like it in the hopes my nephew will play FRLG or BW or something. Children usually see right through that.
AlphaRad defended it in a video I saw recently when he said "Who enjoys wild Pokemon battles?" and I cannot fathom what goes through his mind when he's catching Pokemon in LGPE. You're just swiping with a Joycon for hours. It didn't even succeed in its goal of getting Pokemon Go players to check out the series. It's kind of like "Why did you make the most annoying and unfun part of wild Pokemon battles the only part of Pokemon battles, and why is it now a motion control thing?" They wasted the 2nd Kanto remake to cash in on Niantic's Pokemon game where you can't battle Pokemon.
I've tried Ultra Moon, but so far, I haven't been able to get into it. Maybe someday I'll enjoy Gen 7.
XY and ORAS are about as old as I can go here based on the criteria of this thread, and I agree with what was said above about XY, but they are a bit older. I'd have to buy my nephew a 3DS which is an outdated console because it's not old enough that I can easily emulate it for him on a Windows computer. Maybe I can loan him my 2DS that I recently bought for trading Pokemon in Gen 6, but I don't know.
After typing all this out, I have to add that there's a lot that can change between now (when he's 1 and a half years old) and when I show him a Pokemon game. Maybe TPCi will release a new favorite Pokemon game IMO. Maybe he'll think Gen III looks like a cartoon and he'll prefer that over the modern graphics. Maybe he'll really look up to me, and he'll want to play Pokemon just to spend time with me. (I hope this last one doesn't seem too egotistical. I'm really hoping my nephew will continue to like me as he grows older.) I want to show him cool games! I want to be a cool uncle with a lot of cool stuff!