I don't care how old I am, I will always watch Spongebob.
(Until I can't see, of course.)
My favorite episode... I don't know, I like The Monster That Came to Bikini Bottom though.
There are some episodes that are funny, but some are downright scary (for a children's show.)
Example: The one with the Mad Snail Disease. There were "zombies" all over the place, the town was on fire, people were accusing each other of being a zombie (Patrick had his "I knew it!" moment."... does that sound like something a five year old wants to watch?