The very best spout(of those that I have used) is a Swiss spout.
It has a very good vent, which is the key to smooth fast pouring, and you can empty 5 gallons in approximately 15 seconds. This is a 1" diameter spout that fits American diesel and older leaded gasoline vehicles. It won't fit in the fuel port of American unleaded gasoline vehicles.
I also have heard them called Unimog spouts, but that may be because they were in some of the Unimogs imported to the USA from Switzerland several years ago.
You will have to do a Google search, as I cannot direct you to anyone selling them.
I will try to post a picture before the weekend is over so you will know what to look for.