Paper Mario for the N64 and Super Mario World for the SNES. I think it’s because I found them both at the perfect point in my childhood where they were the first games in their genres I managed to beat.
I had a blast finding every single exit and bonus stage in SMW, and Paper Mario was the first RPG that didn’t make my eyes glaze over (including Super Mario RPG). Plus the characters and aesthetics are still so charming, the whole game gives me a warm fuzzy feeling.