Who would have been the Catholic ruler of England?

Assuming some Catholic claimant actually ascended to the throne after 1701, that very event is likely to have altered his/her choice of marriage partner or that of the offspring thereby changing all subsequent family tree details.

You might as well ask who would be the President of the United States if the Confederacy had won the Civil War.

I think Fresco's point about different marriage choices being made is important

Ok, suppose this. Suppose that the entire non-Catholic Royal Family of Great Britain dies all at once, like a "King Ralph" situation (if you haven't seen the movie, the entire royal family dies in a freak accident and John Goodman, an American, is the closest living heir). Suppose that there were some movement that wanted to restore Catholicism to the British monarchy. Who would they have to scour the family trees from the last Catholic monarch to find to take the crown? I'm just being hypothetical. I'm not that stupid.

Franz Bonaventura Adalbert Maria von Wittelsbach aka King Francis II.
Only as head of the House of Bavaria is "Duke of Bavaria"

(His full title in Germany/Bavaria would be: Franz Bonaventura Adalbert Maria, Herzog von Bayern, Franken und in Schwaben, Pfalzgraf bei Rhein ['Duke of Bavaria, Franken and in Swabia, Count Palatine of the Rhin'].)

Well, his full title in Germany is "Herr von Bayern." Germany doesn't recognize noble titles.

Gordon Brown is considering repealing the 1701 Act of Settlement as a way of healing a historic injustice by ending the prohibition against Catholics taking the throne.
But doing so would have the unforeseen consequence of making a 74-year-old German aristocrat the new King of England and Scotland.
Without the Act, Franz Herzog von Bayern, the current Duke of Bavaria, would be the rightful heir to the British Crown under the Stuart line.
[...]
As for the Duke of Bavaria himself, he is a reluctant heir.
The graduate in economics, who values his privacy, has always laughed off pretensions to the British crown and prefers to concentrate on his modern art collection.
Baron Marcus Bechtolsheim, the president of the administration of the Duke of Bavaria, said: "The Duke generally does not comment on this issue because he sees it as an entirely British question which does not concern him. And he regards it as a purely hypothetical issue.
"Even if this change in Britain happens, it won't change his attitude. All this interest in his opinion makes him smile because, really, he is very happy and satisfied with being the Duke of Bavaria."

Her name is a "clickable" link. Fool.

Thanks for the vote of confidence.

Britain's Real Monarch was a historical documentary presented by Tony Robinson first shown on Channel 4 on 3 January 2004. It has also been broadcast in America and Australia. The documentary discusses the descendants of George Plantagenet, Duke of Clarence and their claim to the throne of England.

The programme based its thesis on the centuries-old claim that Edward IV was illegitimate, born to Cecily Neville by an English archer while her husband, Richard, Duke of York was fighting elsewhere in France. The legitimacy of Edward IV was the subject of speculation at the time, and a document in Rouen Cathedral is presented by Dr. Michael Jones as indicating that Richard, Duke of York, and Cecily Neville were a hundred miles apart during the five-week period when Edward's conception must have occurred. This theorised illegitimacy of Edward would bar a rightful claim for himself and his descendants. This would have the effect of making the heirs of Richard and Cecily's son, George, Duke of Clarence, the "real" monarchs. Such a line would currently be represented by Michael Abney-Hastings, 14th Earl of Loudoun (born 1942), who emigrated to Australia in 1960, married, fathered five children, and currently lives in Jerilderie, New South Wales. Abney-Hastings is a committed Australian republican.