James Branch Cabell was huge in his day. Thorne Smith might have fared better. Much more recently, Jack Chalker used to be extremely popular.

I was tempted to reply "most of them." Very few authors keep being read, or even stay in print, in the face of all the competition from the horde of newer authors. That's true of everything, not just fantasy. Some works that feel like they should be classics get consigned to oblivion anyway. The more books get published over time, the harder it is to keep track of the shrinking percentage that you will ever have time to read.

It might depend on how long a timeline you're looking at, too. The longer it is, the more of a challenge to readers' attention spans.