Dual (#104) immediately came to mind, though Stratagem (#142) is probably just as fun (I have yet to figure it out). Another candidate would be Water Bottle (#111). Although small, I doubt Bind Mender (#126) would be the highest-ranked fun level.
You're right, it is in the top two! It's #2. What could #1 be? I'll give a hint: Unlike CCLP3, CCLP4's highest rated level is one of its smallest.