Corn oil? Olive oil? Peanut oil? Beef or pork lard? Butter?
By “best,” do you mean best-tasting, or best for you?
Duck fat is best but peanut oil is the easiest to get and work with with. Make sure you cook in two stages, first at 325 to cook them, then drain and let sit while you turn up the heat to 375. Then a second fry to crisp and brown them.
Horse fat.
Not kidding.
http://www.hungryinhogtown.com/hungry_in_hogtown/2007/06/the_horse_crisp.html
Try a mixture of about 7% cottonseed oil and 93% beef tallow.
ETA
(That used to be McDonald’s formula.)
How does that translate to electric-range settings (Lo, Med, Hi)?
You need a thermometer, you can’t just put it on high and let it go.
I second the suggestion for peanut oil–it really is the easiest oil to work with for most deep-frying purposes. Of course, animal fats of any kind will add flavor to whatever you’re cooking.
And you can deep fry without a thermometer (I do–there are ways of determining oil temp using a piece of bread, and after awhile you can judge the temp just by looking at how fast something is cooking), but it’s much easier to deal with one.
I should add, duck fat’s (along with most animal fats) smoke point is around 375F, so if you do large batches at a time, you’re going to drop your temp too much and get soggy fries. Peanut oil, on the other hand, smokes at 440F. I generally try to overshoot my oil temps by about 10-15 degrees to compensate for the temperature drop that will occur given the size of my batch and the amount of oil I am using.
Also, as you cook, the smoke point tends to drop a little each time, so even if I use animal fats, if cooking in multiple batches, I cut in an oil with a high smoke point.
The fries they do in Greece, made with extra virgin olive oil, then tossed in salt and dressed with white wine vinegar, taste absolutely amazing. However, it has a low ceiling temperature, so you’d have to blanch them first, and is very expensive. Peanut oil would be best for normal use, IMO.
Just did a little research, and the number is true for chicken, goose, pig, and duck fat. Beef tallow, though, has a much higher smoke point, at 420F.
I really, really want to try fries made with horse fat. Horse meat too, for that matter.
How many minutes for each stage?
Also, what kind of potato is best for frying?
Some recipes I’ve seen online say to soak the potatoes in water for an hour, after slicing and before frying. Why?
Can you even get beef tallow (let alone horse fat) in a supermarket?
My mother used to use beef fat, way back when french fries and beef fat were still good for you ;). They were delicious!
I’m pretty sure you can still get beef fat in the supermarkets.
Whatever they use in my local chipper, dunno what it is though.
I’ve heard multiple times that horse fat is where it’s at. And the cook-rest-cook thing.
Supposedly Belgians make the best fries in the world and, if we labeled things by their origin, we would all be eating Belgian fries. And they used horse fat.
And they serve them with mayonnaise - Mmmmmmmm 
Why are we not talking about bacon??? Imagine frying french frys in bacon fat, then seasoning them with bacon salt. Oh sweet pork lord!
I spent a week in Brussels this summer. I had heard this rumour, and googled around for the best fritkot in the city. Apparently, a big local newspaper had a running competition every year for who knows how long, and this old guy named Jean Pierre, wearing a wifebeater and shorts and puffing on a cigarette ran a little fry shack out of a thirty year old trailer. He was only open for four hours per day and there was a line fifteen people back. I navigated buses and walked through back alleys for fifty minutes trying to find this place, severely pissing off my girlfriend who did not understand my quest for The Perfect Fry.
I ate one bite and was in ecstasy. I am not a frency fry person, but god damn, that was a good fry. I haven’t put a fry in my mouth since that day. It would be like watching your favorite childhood movie and realizing it was poorly made–something magical would be forever taken away by eating an inferior potato.