George III would probably rank in the top 5 monarchs. He was very highly regarded for a long time after his death - which seems very odd to 'mericans who have a different picture of him.
Churchill would work as the leader, but you really need a monarch for the game. As such, Liz does ok plus it allows another female into the game. I am just glad they removed Cleopatra as Egypt's leader (who had not a drop of Egyptian blood in her).
Speaking as one of those "'mericans," the name George wouldn't even cross my mind for English leader.
Funny you should mention Cleopatra, just because the English line has been amazingly resilient to ending. It's pretty impressive how long you can manage down a family tree and keep the next king or queen in that tree, though several "dead ends" in other countries, though no known member of the family still living, revolution, etc.
Anyway, I'd certainly think of the two Elizabeths and Victoria. Those are, after all, the people who are even lend their name to eras. Now, with Elizabeth II still alive, no chance in the world she should get in, as no living leader should.
That leaves Elizabeth I and Victoria. And...I think they got it wrong.
And I base that on the UA. The UA points very directly to the Victorian era. While there is a disconnect for several leaders and UA's, we're not talking about the UA being about or in the time period of some unknown leader (Bismark's UA talks of a time before "countries," let alone "Germany") so why not make the leader, since you get a UA that fits her?
If they wanted to go more cultural with the UA, that would be the ideal situation in which to see Elizabeth, and Churchill could fit in a more warlike UA, but in the end, England has a history of strong and long ruling queens, again, to the point that we can talk about the Elizabethan era, the Victorian Era, and the New Elizabethan Era and know exactly what time period we are talking about.
At least to us Americans, we can't do that with any other rulers, monarchs or otherwise, on that side of the pond.