Time up to review this topic.
I know that this question can be quite subjective. There are many great collectors. Most collectors do not restrict only on one type of collectiables. They may collect coins, medals, banknotes, specimen notes etc. Maybe some of them are well-known for specific collectiables. In fact, coin collecting are meant for Kings in the early times.
Just my own opinion. I think it should be King Farouk. Whether he is a great hoarder or a great collector, one cannot deny his massive collections. I believe all his collections were sold off many years ago in major auctions. I read somewhere that someone actually need to find and record his collecton pieces. Is there a book on King Farouk's collections ? I heard most of his pieces exists as one or two pieces in the world.
I came across a pair of China gold medallions which was struck specially for visits by China envoys to Egypt ( I don't have the details on that ). The pair of gold medallions were in King Farouk collection. They are relatively huge. I actually hold them in my hands to examine it. That was two years ago when a friend won the pieces during an earlier auction. As these are a different level of collection arena from mine ( they cost many thousands and million in value ), I did not ask my friend for any details and also do not have that kind of money to keep them as my collection.
With my brief encounter and limited knowledge on King Farouk's collection, this topic was created.
Thanks for everyone contribution !!!