Current: Bonnie and Neddy. I just love that it's a full episode about the gum people and that because before it possibly no one really knew about mother gum apart from the people the who play "Adventure Time explore the dungeon BECAUSE I DON'T KNOW" (like me).
Old: The old episode that I personally enjoyed was "What was missing" in season 3. It was so amazing. I liked that all the main characters got to spend quality time together and... SING!!!