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.
Hate on me all you like but Fortnite would be in my top 10.
It isn't without it's down sides but it's clear some people criticizing haven't played it recently. For example ranting about low quality graphics is ridiculous since this season the graphics are much more demanding. Though the core of the game is the same, the game play is quite varied when they get into the themed events and limited time modes. I already miss the light sabers from the recent Starwars LTM. And the Marvel LTM was pretty good too. The challenges can get repetitive and grindy.
Still, it's immaginative, multi-player, multi-platform and you can play for free. I haven't given Epic a cent and I have reached level 100 since season 7 and am at 330 on S2Ch1. (Disclaimer: My kids both bought Save the World, and one spent $15 on a starter pack at the end of Season 6.).