Actually, His Imperial Majesty the Emperor Akihito of Japan is the highest ranking Monarch in the world, as an Emperor outranks a King. A Kingdom never outranks an Empire (even if this Kingdom happens to have a Commonwealth). In regards to who is the most well known and bublicised Head of State, then without a doubt Her Most Gracious Majesty the Queen Elizabeth II takes the cake. When it comes to European Monarchs I believe that Elizabeth is most certainly the highest ranking sovereign and holds one of (if not) the most symbolic position any European Monarch ever could or shall (This is also contribued to by the fact that the Queen is also Head of the Commonwealth and as such is Queen to three of the 15 other indapendant realms outside the United Kingdom (which is also a Commonwealth Realm). Those three countries being Australia, Canada & New Zealand.
"MII"
As for who shall be next to succeed to a throne. I really cant say.