If I can level up, find new gear, kill monsters (and then rinse and repeat for 100s of hours), it is for me. Unfortunately I don't seem to make time to game consistently anymore (bought a 4080 1.5 months ago for Starfield and still haven't installed it...)
My all-time favorite, probably because it was my first online RPG was Phantasy Star Online (1 and 2) on Sega Dreamcast. An upgraded version was released for Xbox One and I actually put a decent amount of time into it a year or two back so it got me deep.
Another major time sink was WoW for at least 3-4 years. I'd honestly try it again but it would definitely piss off my wife at some point and kill my other hobbies, its so time consuming.
And then Morrowind/Oblivion/Skyrim...loved them all.
A lot of good stuff on this list. Halo and Diablo II with buddies back in the day was definitely great times.
Hey, that's a tough one because there are so many great games out there! But if I had to pick one, it would probably be "The Legend of Zelda: Ocarina of Time." It's got a special place in my heart for its amazing storyline and the countless hours of fun it's brought me.
By the way, if you're a fellow gaming enthusiast and want to make some extra cash, you might want to check out apps that pay you real money. They're a cool way to earn a little something while doing what you love!
Souls games, easily. Dark Souls 1, Dark Souls 3, Demon's Souls, Bloodborne, Sekiro, Elden Ring, etc.... They are just so good. Even some of the Souls-likes not made by From Software are great, like Nioh 1 and 2, Lies of P, Mortal Shell, etc.
Also a big fan of 2d "metroidvania" games, like Hollow Knight, Ori and the Blind Forest/Ori and the Will of the Wisps, Super Metroid, Castlevania: Symphony of the Night, etc.
Others: The Legend of Zelda: Ocarina of Time and Breath of the Wild, Metal Gear Solid 1 and Metal Gear Solid 3: Snake Eater