I'm not able to pick anything that can qualify as "best ever", but I can say what I feel have been the most influencial to me.

Another Story
This pack is very short, and there really isn't a whole lot of gameplay. But what this pack has makes up for that. Another story has extremely innovative concepts in storytelling and jcs usage, an actual plot, and everything seems to go together perfectly. This level pack showed me that I could make something more than just a sequence of levels with JCS. You might also notice that a lot of ideas first introduced here are used in my episodes, as well as a number of homages.

Tomb Rabbit
This level pack is too hard and has a whole bunch of problems, and really isn't all that fun in some parts. Still, it has some concepts which make it pretty amazing. To me, the most important one is the central cavern, and how you visit it many times. This gives the level pack a more immersive feel, as it makes it so that once you leave an area, you haven't left it behind forever. The level pack becomes less of a straight path, and that makes it interesting.

Blur's level pack demo, Moonblaze's pack (I forget their names)
These ones just have cool level design.