What do you mean by "greatest success in selling"? Do you want it sold fast or sold for the most money?
You can put it up for a no-reserve penny auction. Guarantee it'll be sold by a certain date, but probably won't get as much as you were hoping for. Or you can list it online for how much you want to get, and sit around for as long as it takes waiting for a buyer.
Facebook Marketplace and the dedicated facebook 71-73 and/or mustang groups is the best option for where to sell a car that reaches the most potential buyers, while paying zero fees. Its up to you to take good pictures and make sure the car is listed correctly with a good description.
Then there's places like ebay and bringatrailer. They get more traffic which could mean a higher sale price, but will expect a cut of the profit. BAT will help you with the listing.