Probably any NLT game, I played them all a few times. Or what about The Artifact Part I + II + III. That game I played like 3-4 times over the years, still like it a lot. Would love to see an updated version, with higher resolution graphics in it...
Most played must be Minecraft, bought it from mojang before mods were a thing and even ran a few servers myself back in the day. Still play it every now and then.
Time played: YES.
I don't really know which would be the second, but 50 hours is nothing.
My average Fallout game(any version) play-through is easily 100h+ a long one can reach 300h. I've beaten all of them many times and the same goes for elder scroll games..... So probably one of those is second.
Started playing poe during their beta(or was it an open alpha? can't remember, but there was a buttload of bugs and the condition for playing it was reporting bugs), still play it a few hundred hours each league.
Started playing lol during the beta. Dropped it long ago, but played easily a few thousand hours.
Started playing dota in warcraft, but really got hooked when dota2 was launched, at least another thousand hours.
Honestly I am not sure on mainstream, but think it's Age of Mythology: The Titans, I think I have around 20000+, I started playing around 1999 and for close to 15 years I played it for a couple of hours daily, after that I played it from time to time to this day, as for adult games, have no idea, as there are some Japanese VNs that are really big that go for about 80+ hours, and I have played a lot of them
Mainstream game: I think I've played FFX around 300 hours (blitzball, yay! ) when I was a kid.
Adult game: that's a tricky one. Very hard to tell since I usually play in a very fragmented way because of the Patreon model Recently, I've played for the first time Stray Incubus and it was pretty long to get to the current content ^^