I think the most popular name for European monarchs (defined herein as kings or emperors) has been Charles. Let's see: two kings of England & Scotland, ten kings of France, one king of Portugal, four kings of Spain, seven Holy Roman Emperors, one (beatified) Emperor of Austria-Hungary, nominally sixteen (actually ten) Kings of Sweden, two kings of Romania, one king of Württemberg. Did I miss anyone? Oh yes, there's Naples & Sicily which gets complicated because some kings ruled both with different numerals and then the last King Charles (VII/V) subsequently became III of Spain. But if you're ever in a situation where you have to guess the name of a European monarch, you have better odds with "Charles" than any other particular name.
Louis, especially after Louis IX was canonized, was extremely popular in
France, going all the way up to XVIII (Woody Allen's character in
"Small Time Crooks": "how high do the 'Louis's go? Anyway, it's a top
Louis."), but in other countries, we have four Holy Roman Emperors, two
kings of Bavaria, one king of Portugal, and one short-lived king of
Spain. Not bad I suppose.
Edward unsurprisingly has been the most
popular kingly name in England; the numbering only goes up to VIII, but
there were additionally three Kings Edward before the Norman Conquest.
But the name never really caught on on the Continent; just one king of
Henry is another very important monarchical name;
obviously, eight Kings of England. Four Kings of France, seven Holy
Roman Emperors, four Kings of Castile.
We mustn't forget my own
brother's name, William. Four kings of England (and God willing, one
future), four kings of the Netherlands (though the present one insists
on going by "Willem-Alexander"), two German Emperors, two kings of
Francis: two Kings of France, two Holy Roman
Emperors, and two Kings of the Two Sicilies. (Of course now there's also
the pope, but I'm not dealing with popes here.)
the name of seven kings of Spain, three Holy Roman Emperors (a fourth
was king of Bohemia & Hungary but predeceased his father so never
became Emperor), two kings of the Two Sicilies, one Austrian Emperor,
one king of Romania, one king of Bulgaria.
Philip: six kings of France, six kings of Spain (including the present one), the present king of Belgium.
Leopold: three kings of Belgium, two Holy Roman Emperors.
Maximilian: two Holy Roman Emperors, two kings of Bavaria, one Emperor of Mexico.
Joseph: one king of Portugal, one dubious (Napoleonic) king of Spain, two Holy Roman Emperors.
Alexander: three Kings of Scotland, three Emperors of Russia, one King
of Greece, one Prince of Bulgaria, two kings of Serbia/Yugoslavia
(though the second did not use "II").
John: one king of England
(not a great success), six kings of Portugal, two kings of France, one
king of Saxony, one king of Denmark.
Frederick: three Kings of Prussia, nine kings of Denmark, three Holy Roman Emperors.
George: six Kings of Great Britain (III & IV were also kings of
Hanover), one additional king of Hanover, one king of Saxony, two kings
Peter: four kings of Aragon, five Kings of Portugal (the 4th of whom was
also I of Brazil), two Emperors of Brazil, three Emperors of Russia, two
kings of Serbia/Yugoslavia.
Otto: four Holy Roman Emperors, one king of Greece, one king of Bavaria.
Harold: two pre-Conquest kings of England, five kings of Norway (including the present one).
This isn't really in any sort of order anymore, is it?
Most other names seem to be associated primarily or exclusively with
one monarchy (e.g. Christian in Denmark, Gustav in Sweden, James in
I'm just typing this, without doing too much research
to refresh my memory, on a rainy afternoon for no reason really. Hope
someone enjoys it.