Definitely Pokémon Red, I was sitting on our typical 90s beige colored couch watching cartoons when my grandma and grandpa came in and gave me the game and a green Gameboy Color as a present, it's one of my clearest memories from that time. I got it pretty early in comparison to many others in Denmark, I was at least the first to own it at my school, so this was before it really boomed across Europe. I brought it with me to school one day and it ended up spreading like the plague, a month or so later everybody was playing Pokémon on their Gameboys, and since I got the game earlier than everyone else everybody would ask me about things constantly. Starting a fad at your school as a kid is a really good memory to have, I remember all the crazy things like Mew being under the truck and finding out about the Missingno glitch, our save files getting corrupted, etc. It actually continued for a really long time too, the majority of our class was into Pokémon up until and including Ruby and Sapphire, with the drop-off happening sometime around Emerald where people simply grew out of it.
I still play Pokémon Red sometimes today, it's the one I'm most comfortable doing challenges like nuzlocke with, and while I certainly acknowledge its many many flaws it'll always be my go-to game because of its nostalgic value.
