Who Is 'the self-styled Ruler of Middle-earth'?

So, re-reading the Foreword to THE LORD OF THE RINGS last night I was struck by a familiar passage I realized I haven't fully thought through before. After pointing out that his book is not a roman-a-clef allegory of wartime politics of the World War II era, Tolkien gives an alternative summary of how things wd have gone in his book had that been the case:

The real war does not resemble the legendary war

 in its process or its conclusion. If it had inspired or 

directed the development of the legend, then certainly

 the Ring would have been seized and used against 

Sauron; he would not have been annihilated but 

enslaved,* and Barad-dur would not have been 

destroyed but occupied.  Saruman, failing to get 

possession of the Ring, would in the confusion

 and treacheries of the time have found in Mordor 

the missing links in his own researches into 

Ring-lore, and before long would have made 

a Great Ring of his own with which to challenge 

the self-styled Ruler of Middle-earth.  In that conflict 

both sides would have held hobbits in hatred 

and contempt: they would not long have survived 

even as slaves.

(LotR.7)

Although Tolkien does not name this 'self-styled Ruler of Middle-earth', he offers some clues as to who he, or she, might be.

First, we know that it's not Sauron, who has been defeated and imprisoned.

Second, we know it's not Saruman, since he's acting in opposition to the Ruler as his or her rival.

To this I wd add that the Ruler wd have to have (1) an opportunity to seize the Ring and (2) the stature to be able to wield it**

I therefore come up with a list of seven candidates:***

Gandalf

Aragorn

Elrond

Galadriel

Boromir

Faramir

Denethor

Of these, I think Boromir and Denethor are the likeliest: the ones most in favor of using the Enemy's weapons against him. Plus of course Boromir actually attempts to steal the Ring, and Denethor makes it quite clear that he wd have used the Ring had it been in his possession. 
On the other hand, that not even Isildur cd master it cd be used to build an argument that sufficient stature requires the new owner of the Ring to have been Maiar, not Mortal. If so, the list of candidates shrinks to just three: Gandalf, Elrond, and Galadriel -- all of whom are already Ringbearers of the Three Elven Rings.
Set against this is the fact that Sauron seems to most fear Aragorn, once he learns of his existence, and you'd think the Dark Lord wd know better than anyone else who posed the greatest threat to him in terms of pure power politics.
So, though I cd make a case for any of these seven, Denethor gets my vote of likeliest to turn into the next Sauron, with Boromir close behind.

--John R

*This of course wd have been repeating Ar-Pharazon's mistake

 **or it wd just wind up (briefly) in the possession of another Gollum. 

***I exclude Bombadil from this list, for reasons I assume will be obvious