Legit, Pokémon Yellow (original GBC release). I adore Johto and it will always be the core series game closest to me, but I found that upon replaying Yellow, there is an undeniable charm to it. It felt like taking a direct hit to the nostalgia funny-bone and I could help but be enamored with the game, with Pikachu, and the anime plot beats it follows.
During those replays, I always felt like Pokémon Yellow went the extra mile. Little things like Charizard being able to learn Fly, the addition of few updated sprites, Jessie, James, & Meowth, and more anime accurate ace Pokémon the Gym Leaders use.
But obviously, the most fun and replay worthy moments are all about Pikachu. I adored that he would follow behind you and I remember constantly talking to him and hearing that 8-bit "PIka" sound that changes in tone depending on his mood. I loved making notes about the items/events where that sassy, defiant lil' rodent grew more accepting and friendly, and would eventually develop a trusting, loving bond with the player. Oh! And finding all the actions/locations where Pikachu would display a unique mood — like falling in love with Clefairy and wearing a hat while fishing (which isn't an emotion but COME ON).
I know this mechanic was made to mimic the anime, but they didn't need to give Pikachu so much personality. Pikachu was so interactive and so emotional, it was impossible to not find yourself truly, emotionally connected with those pixels. Experiencing all the highs and lows. The idea of earning a Pokémon's respect, or distain, gave Pokémon Yellow endless replayability for me.
Also the Surfing Pikachu mini-game of course!! My lil'electric boi was a champ! Those moments of genuine pride I felt, or shame of losing, the worry when Pikachu was poisoned, the preciousness of him falling asleep to Jigglypuff's song, and the absolute delight of knowing I would always have a best friend in Pikachu. That's what makes Pokémon Yellow such a fun and unique journey worthy of numerous revisits. ଘ(੭ˊ꒳ˋ)੭✧