I don't know if there is a canon explanation for this, but I've been wondering about it lately. It has been stated in games featuring them that both Richter and Julius are supreme amongst vampire hunters, but this is the kind of statement that could be talking specifically about the time in which they lived. They were supreme during their lives, but how do they stack up in comparison to their descendants and forebears?
To my mind, it seems the only way to accurately judge this is to see how well each one fared against Dracula. Since the only foolproof barometer of this I can think of is how often they fought him, this really only leaves Simon and Christopher up for consideration. Both defeated Dracula twice, and they did it alone both times. Personally (and part of this could be my bias against The Castlevania Adventure showing) I'm inclined to believe Simon is stronger because he was able to travel vast distances to assemble Drac's bodyparts, and then defeat Dracula, all while he was dying from the curse.
What do you think?