Which oils or fats make the crispiest roast potatoes?
I think it's more about technique than what fat you use when you're looking for crispy outsides (I'm sure there is at least one question here already that has the technique answers). 

About the only solid advice I would give is that you need to find a fat that tastes good to you, or is neutral if that's your preference, and make sure the one you use won't burn at the temperature you're using. 

Beyond that, it's all about taste and technique.

edit: I looked for questions that might have solid technique answers, but didn't find any. So I looked out on the greater web and found some advice from Cook's Illustrated. Crisp Roasted Potatoes

It doesn't have the recipe for free, but the advice is good:
Parcook at a fast simmer, get a little rough with the parcooked potatoes to rough up the outside, choose the right potatoes (they liked Yukon Golds).