ohhh boy, that's a toughie.
if we're talking about total time spent, technically...
spore. i have over 600 hours currently on steam for it, but my computer refuses to close it and has been for about a week now. realistically, i've spent about 120 or so hours on it; just the steam version though, which i've had for about two years. to be honest, it's the nostalgia factor.
on my old origin account from when i was little? i can't dig it up as of now, but that's definitely between 300-1500 hours of spore in the span of a bit over 3 years. talking a single game, technically my "favorite" would be spore.
now, on favorites generally? well, like i said, a toughie. i'm just gonna go by the three main systems i use. time for sperging
as for PC, it's definitely
lisa: the painful, which i discovered late last year. it's definitely not for younger audiences, but it deals with tons of themes including addiction, sexism, obsession, and trauma, some of which hits pretty close to home for me. it deserves wayyyy more attention than it gets, in my opinion. there's lots of choices and it doesn't want you to go one route or the other. unlike other games with routes like undertale, which try to urge you to the "good" route, there's no real push to either side here as humanity has pretty much completely given up. you can sacrifice your party members, and i mean actually sacrifice them, you can give them away as a choice. as an example, the first choice of this kind is either the first member you have in your party, or one of your arms, hindering combat. the gameplay is typical for an rpg maker game but it has its own flair that makes it unique.
as for PS4, i have to be an absolute normie and say
persona 5. the gameplay is super fast paced yet there's a long story and lots of links and interpersonal relationships to choose and further, giving it a lot of replay value despite being over 100 hours long. though i'm pretty over-critical of things and the writing is pretty subpar at times to someone who's extremely stingy about that kind of thing, but chances are you won't really notice unless you're big into writing or analyzing stories.
3ds is super tough since i have a very large library of games for it, but at the moment i'm super fixated on new leaf. i don't feel like i have to explain this one considering the forum i'm on tbh
honorable mentions are mother 3, hylics, pokemon mystery dungeon: explorers of sky, and danganronpa.