For myself, the answer to this depends very much on various factors. Who is most loved, most powerful and influential, who did the most to change and positively reform the monarchy and the countries over which they reigned.
i am not entirely sure I'd like to say who is the greatest. Of course our current Queen is a much loved monarch, perhaps one of the most adored we have in a very long time. In terms of the other factors of which I mentioned, I'm not sure she scores very highly. However in context of the times, a monarch wouldn't be able to conquer other countries and have political and sociological influence. Of course she has been a very good monarch in many ways, being the saviour of the monarchy in several instances, and being able to adapt successfully during the darkest times.
Richard The Lionheart, Henry VIII, Charles I Mary and especially rather ludicrously John are bizarre choices for the poll. All of which were horrible rulers with very few redeeming factors.
I am uncomfortable saying who I think was the greatest. I feel that saying one is greater than another, may be to the detriment of others. Some of the greatest in my view are Charles II, Elizabeth I, William The Conquerer (although his treatment of his subjects leaves much to be desired), and Victoria amongst others. All for widely differering reasons.
"I know I have but the body of a weak and feeble woman, but I have the heart and stomach of a king, and of a king of England too."
— Queen Elizabeth I