It's a tough one, because you have to take into account how times have changed with each monarch. What made a good rule in the 11th century is very different to the 19th century.
I am very fond of our current queen, Elizabeth II, but she has not got the same kind of power over the land as was the case in the past. I think her father was a brilliant King, the way he dealt with the war, his stutter, and the fact he never expected, nor wanted to be king.
I'm interested in the Victorian era, so Queen Victoria is my favourite to learn about. I think she was a good queen, but a very poor mother.