Can't we all just agree that things are based on situation?
Looking at that list, I'd put Luthien above Gandalf. Putting all of Angband to sleep is enough to qualify her for a higher position. Also, since Tolkien set the situation at Minas Tirith to appear as a clash of equals, I would put Gandalf the White and the Witch-King side by side. This is a list, yes, but due to no conflict at the gates of Minas Tirith and the way the passage was written, the best choice in my opinion would be to list them as equals.
In the end, however, I would agree that the situation is too important a factor. The only characters that can safely be put on the list are Eru and Tom Bombadil. Even that is unsure.