It's Skullgirls by a mile, at over 1600 hours. There's always some way to push your game to the next level, and there's always a strategy and combination of characters you can put together that's effective and no one has tried before. Next is Guilty Gear Strive, at over 700 hours, and the Roman Cancel system is so deep that there's always room to be clever with it.

Looking at some other games I've played a ton, besides fighting games, I could probably sink hundreds more hours into Baldur's Gate 3 and Mercenary Kings if they ever got expansions or sequels, but I don't think those things are going to happen.