achieving a mild crusty on top of mac'n'cheese...

[Bogie]

Should I just propane, or broiler, or what? Or not bother?

[K Frame]

I use buttered bread crumbs.

[MillCreek]

Shredded extra sharp cheddar and run it under the broiler.

[kgbsquirrel]

I'd combine both of the above toppings, because why not get both?  I hear crumbled up ritz crackers work well for the bread crumbs.

[K Frame]

"I hear crumbled up ritz crackers work well for the bread crumbs."

They do.

Keebler Original Club Crackers (the ones in the green box) also work well.

[charby]

I bake mine covered in foil, remove the foil in the last 15 minutes and broil for 5 minutes if the top isn't browned like I want it.

[French G.]

I am becoming partial to buttered bread cubes.

[T.O.M.]

I mix Ritz cracker crumbs with parmesan cheese, and spread that on top, then hit with the broiler. Melted butter is a good idea to add to it.  Then again, I'm a heathen, and add shredded mozzarella to the cheddar with the Mac.  Cuts some of the sharpness of the cheddar, and gives the cheese that great stretch.

[charby]

Panko also makes a nice crunchy topper.