TES Oblivion and Sonic Adventure 2. SA2 was a childhood favorite I got very good at, and the extra mission, game modes, and pet sim aspects makes it fairly re-playable without having to start a new game file. Oblivion is just whacky and fun, and if you know how to exploit mechanics you can get up to some pretty crazy stuff in a matter a minutes after starting the game. While grinding skills could be seen as... Well a grind, it's a grind I personally enjoy. Both games let me dive straight into what I'm feeling like doing, and reward mastery (even if one of them is cheese mastery).