My personal opinion is that Nihilus, in terms of raw power, was > anything.
However, he paid a price far in excess of what he gained; he was, as far as i remember Kreia putting it, "just hunger". No desire, no passion, barely intelligence, simply all-consuming, himself included.
He was more of a force of nature than a force of being. The Dark Side taken to an extreme, aptly likened in this thread, to a black hole.
I think, that if any of the sith in the empire was confronted with the pros and cons of Nihilus... That none of them, power hungry as they are, would be willing to pay that price. They might be enticed and consider it, but confronted with the full scope of it, I sincely doubt any of them would take the deal.
How is that old highlander quote again... "Better to burn out than fade away?" I believe that applies very much in this case ^^
It does beg the question... What the H was in his Holocron? And how/when did he make it? A force of nature doesnt exactly keep a diary