I was going to say replaying but I feel like that limits the question to games like Prey or Fallout New Vegas that have endings and games like The Sims 2 or Cities Skylines where you can play indefinitely end up excluded.
I’m not really referring to games like League of Legends where you’re coming back every month. More so games where you stop playing for an extended period of time.
Portal 2 is definitely the one I pick up regularly, but specifically for the Perpetual Testing Initiative. I’ve already played the main story enough times, but dropping in for a few really well-made user-created levels with a little bit of new Cave Johnson dialogue is great!