Dota 2 is the most I have logged at around 3000 hours, but I don't have any friends to play with and don't want to deal with randoms, so almost all of it is against bots. Even though the bots are garbage, there's still so many creative, silly ways to play that game and ridiculous builds that are fun to try and see if they work.

Super Smash Brothers (64 and melee) is probably the most of all time though. Any time I pick up a new 2D game, I find myself comparing it to SSB in terms of mechanics. The way you control your characters in that series is the most innovative and compelling concept I have ever seen in a game. If I ever have the time to make a game myself, I would probably heavily model it based on that control scheme.

I also have to give a shoutout to Streets of Rogue, which is a fairly obscure rogue-like that deserves a lot more attention IMO. Another very replayable game with many fascinating play styles and ways to complete missions. It's on Steam and it's fantastic, I would highly recommend it.

Enter the Gungeon is another game I have high playtime on. It took about 40 hours before it started becoming fun, but man, it's awesome once you finally figure out how to properly dodge enemies and make the most of your weapons and items.