I think I'm the only one that likes IV the best. The whole wumbo theory makes me laugh just of thinking about it: "I wumbo, you wumbo, me he she wumbo. Wumboying. Wumbology. The study of Wumbo. It's first grade Spongebob!"
The episode overall makes me laugh the best. And V is really cool, I loved to see them as superheroes, but it wasn't as funny as IV, IMO. III has a lot of funny quotes but I have it almost tied with VI. Those are my top 3.
BTW:
IV- shrink ray
V- Superfriends
VI- Movie