I understand nostalgia may (and likely will) play a big part in this, but that's fine.
For me it's Breath of the Wild. I know this may not be a surprising or unique answer, but as someone who has always been a big Nintendo fan, I never really got into the Zelda series. I'd always get 5-10 hours into a game, get lost in one way or another, tell myself I'm coming back to it and never do.
Considering my expectations were somewhat lowered as a result, first playing Breath of the Wild was an absolute treat. The fact I could do whatever I wanted at my own pace without feeling a sense of pressure or anxiety artificially induced by the game along with knowing I could literally go anywhere without the hindrance of invisible walls, weird geometry or advanced mechanics made me feel like being a kid again in the same way I'd always explore every nook and cranny of a level in Banjo Kazooie to avoid the harder boss fights or explore every inch of a Pokemon overworld looking for hidden secrets, only now as an adult and years of gaming experience behind me to see me through the harder bits I would have avoided as a kid.