A bit of a caveat is in order here, I suppose. First and foremost, I was trying to spare the professor from reading another "I love Jeb Stuart/Custer/Forrest" essay. Second, I was also looking at all aspects of the duties cavalry is expected to perform - intel gathering, screening troop movements, guarding supply lines, raiding supply lines, effectively working within the larger scope of a campaign, etc. And yes, Western commanders had a much much larger area of operations than did those in Virginia.
I was leaning toward John Buford for this answer, but honestly don't know enough about the man to have done an essay about him off the top of my head. But in Earl Van Dorn you have a bit of a unique situation - no other cavalry commander worthy of consideration for "greatest cavalry commander" (yes that is a deliberate slap at Sheridan - I despise the man) had the experience of being an independent theater and army commander. Van Dorn also has that whole Sidney Johnston-effect going for him too, in that he was removed from the picture too soon to know what he was capable of - as a cavalryman at least.