Hard to pick just one...

Inferno is the most artistically impressive. For a cartoony platformer, it's a rather convincing depiction of hell. The flesh-trees really freaked me out when I was a kid.

Castle is probably the best balance between easy-to-use and good-looking. In terms of giving a newbie something to work with in JCS, it gives you the easiest means to produce something that looks cool.

Holiday Hare is the most versatile one. You've got outdoors, indoors, houses, and a pretty good set of tiles (for one drawn by Stadler) to connect them all together in meaningful ways. There's pretty much all kinds of destructible blocks, vines, poles, a stump for a shootable pole, pinball events look okay.