Pinkie pride probably.
It was a superb episode to display all sorts of emotions Pinkie goes through. Weird Al took a near perfect guest roll for a well handled guest pony. The songs were great, the dynamic was good, it had Pinkie's cutest gags in there, the resolution was amazing and the climax was near grand finale levels great. It was both really funny and very heartbreaking.
(second up, not surprising Too many Pinkie Pies)