As mentioned already, you'll get 50 different answers to this question. Here's mine...
If you're a relative novice at crappie fishing, use the KISS(Keep It Simple Stupid) method.
Don't bother with complicated leaders and line combos. Just get a good quality High-Vis line in 6-8lb test, and tie directly to your jig(s).
My thinking on this is simple: Crappie fishing has a lot to do with feel. The best crappie guys out there can not only find fish, but they know a bite when they feel it (or see it...hence the High-Vis line). Putting a leader, swivel, etc between you and your jig isn't helping your "feel" if your new to the "thump".
If you are a novice, you're going to get hung up a lot too. The mono is easy to break off and re-tie. As you progress as a crappie guy, maybe you'll decide to change over braid...but I doubt it.
I personally use a line I don't see many people mentioning, but it's probably similar to Mr. Crappie High-Vis. Very low memory.
Sufix Elite High-Vis Yellow 8lb test