I was not lucky enough to ever play the spin-off games for the GameCube, so for me this one is gonna be pure nostalgia. My first ever Pokemon game was LeafGreen on the Gameboy Advance SP, and although I never fully beat the game until a few years ago, I used to play LeafGreen all the time. It was the only Pokemon game I had until the Nintendo DS and Pokemon Diamond/Pearl came out, but I remember bringing that game with me on every car trip for years when I was a kid. The best memory I have of that game was bringing it to a special bar/restaurant that my family always goes to for family dinners, and we used to always sit at the same large table (the only table they had bigger than a table for 4) that was also right by the window. My brother (who had FireRed) and I would just sit at the window and play Pokemon the entire time.
i love Diamond , the first time i ever got to trade pokemon because of the wireless function, i ran all over town trading pokemon with my cousins and friends who had the game xD