If 1 and 2 were native to the GBA people wouldn’t say this. 3 is my fave because it came out at the right age, but 2 had better mechanics such as the day/night cycle.
The day night cycle is about the only mechanic (and headbutting trees) in gen 2 that 3 lacked, but 3 made up for those lacking features with all the other cool stuff like hidden bases and a much richer story (and better puzzles in the gyms[looking at you lt.surge gym...]) Also, gen 1 got a remake on the gba. Gen 1 has it's charm, i grew up playing it but it's a buggy mess and has lots of issues(psychic types being absolutely unstoppable due to basically no weaknesses being programmed in)
The berry growth(and ability to plant specific berries). Better bicycles.. way better balance for mon types. The addition of "abilities" was pretty big too, imo. Need i go on?
Berries and bikes, were kinda just there. I can see how people liked them though.
abilities just made competitive play worse because Nintendo actually expects people to grind out catching/breeding perfect EV and depending on the moon with hidden ability.
I guess we just disagree on what additions are good.