I always enjoy going back to Pokémon Yellow, where it all started for me. It's easy to emulate, thoroughly nostalgic, and just fun to revisit and muse on the beginnings of the series. My last replay was the one I did to capture screenshots for the article I wrote on the Mew trick for TCoD, and I had a very good time with it, though I stopped off at Saffron; vaguely want to go ahead and finish that playthrough.
Something that's fun about R/B/Y is how there were progression puzzles that were a lot subtler than what you get in the games today. Nobody tells you you can get drinks from the vending machine to give to the thirsty guards; you just need to hear the guards are thirsty, see that drinks are being sold somewhere and put two and two together yourself and try it. Nobody straight-up tells you you can use the Poké Flute to wake Snorlax either; Mr. Fuji just tells you it'll work on any sleeping Pokémon, and you need to infer and try it. As a kid, you get to feel really clever for figuring these things out! But, unfortunately, they also meant many kids just got stuck there, so I believe the remakes streamlined these things to be easier but also have less of that sense of discovery.
Likewise, I remember feeling clever for noticing the little dots on the floor indicating where the invisible walls were in Koga's gym. That feels good! The walls being actually invisible in the remakes ironically takes away from the original appeal a bit, because you don't get to have that moment where you realize you can tell where they are.
