Postal 2. The game mechanics and open-world flexibility have aged amazingly well, it's still very funny, and I love the way the game's level of violence firmly depends on the player's actions.
Plus the Postal Dude's petition to make whiney congressmen play violent video games is needed more than ever.

On Android I miss Spaghetti & Marshmallows, where you had to build towers out of said materials. That was a wonderful game with great physics but sadly only runs on very old phones.