It's easily Outer Wilds for me. While it's a game I played for the first time just three months ago, it's one of the best games I've ever played, and one of the thoughts I had after finishing it is that I wish I could experience it for the very first time again in the future. Part of what made the game so phenomenal for me was being able to go into it blind and be filled with a sense of wonder, a sense of discovery, a sense of accomplishment, wanting to keep playing more and more so I could explore the entirety of the world and unravel all the mysteries the game had to offer and figure out how to reach the end of the game. It captured feelings I haven't felt when playing a video game in a very long time, and it does honestly make me sad that I know I'll remember too much of the game in the future to be able to have those same feelings again when replaying it, so that's what makes it the perfect answer to this question for me.
Honorable mentions would go to some games I've played numerous times over the years, just because it'd be fun to get a chance to play them all over again for the first time: Legend of Zelda: Majora's Mask; Skies of Arcadia: Legends; Metroid Prime; Pikmin.