Xenoblade Chronicles, I bought this game on a whim one day purely b/c I wanted to play a new wii game [I knew about it from operation rainfall but I wasn't into jrpgs at the time] and it was the best nonchalant game related decision I ever made. For maybe about half a year, it just took over my life, I would skip out on hanging out w/ friends just to continue playing this game. I remember actually thinking 'I can't believe a game this good exists'. What also made the experience special was that it was still a niche game at the time I played it, so I very much got the feeling that I stumbled upon an amazing hidden gem that few others really know about. I just poured over Xenoblade and it basically made me pledge my loyalty to monolith soft, so of course I had to go back and look at their past works. Part reason why I got a PS2 in 2015 was to experience the Xenosaga trilogy on my own [that and I just wanted to play certain childhood games again]. What made me stop playing was my brother had deleted my file for that game, b/c I was using his wii at the time since mine couldn't read discs, and I was freaking devastated. I had already beaten the game at least but man, I really wonder how much longer I would have kept playing if that didn't happen. Since then I've been playing a variety of games and rpgs in hopes that maybe one day I can recapture that "Xenoblade experience", and so far it hasn't quite happened but that's fine.
2 other games that I just need to mention are Metal Gear Solid 3: Subsistence and Grand Theft Auto: Vice City, games that I sank a lot of time into and just take me back to my adolescence and childhood when I play them. Vice City is probably my 2nd most played video game if I were to combine all the hours from childhood and nowadays.