It’s easily The Legend of Zelda: A Link to the Past.
It has everything I could ask for in a game: Sword fighting. Magic. Secrets. Dungeon crawling. An alternate dimension. Side quests. Different tools and items. There’s enough content that it feels fulfilling to complete it. Peak art. Peak music. NPCs don’t talk too much, and there are just enough of them to make the world feel alive. Bosses.
There is definitely a nostalgia factor, and I was into playing the TCG with the actual cards too, so that helps. It’s been a long time so I’m not sure if it would still hold up.
From the main games I have played Red/Blue/Yellow, Gold/Silver, and Emerald. I think they are all pretty close in terms of favourite, but I’ll say Blue because it was my first and had a huge impact. I remember battling other people using a link cable at school.
I think those games all have the same problem. The battle system is too slow by today’s standards. Random encounters can get annoying (thanks, ZUBAT!). It’s not just exclusive to Pokemon. Other series of the time haven’t aged as well for the same reasons. Back then it didn’t matter though.
Pokemon Stadium deserves an honourable mention. Seeing the Pokemon animated in 3D was amazing at the time. The mini-games and the gameboy pack that connected to the N64 controller were great too.
Pokemon Snap - meh. It was the type of game you play at the demo booth while waiting for your parents to finish shopping.
Pokemon Trading Card Game - the gameboy game and the physical card game were both peak. I feel like the gameboy game was slept on by many, but I loved it.