The Binding of Isaac. Its hard to tell exactly, but I have about 600 hours across the versions on steam. I also have a handful of hours on 3ds and a handful of hours on Switch.
Its just a game I can keep coming back to. Every time its something a little bit different. The skill ceiling is high. The moment to moment gameplay is great. I do wish it was a touch less scatalogical just so it would be an easier recommend to others.