RZA peaked higher but he fell off harder. Madlib has been much more consistent despite what people say about his quality control (which, to be fair, has some truth to it).

Also, RZA's classic albums have as much to do with the talent he worked with as much as the brilliant production he netted. Classics come from both ends, and RZA did his part just like Madlib did when he worked with equally competent MC's, most notably with DOOM.

I think it's really close and it depends on what you value in a producer, but Madlib is still going strong after 20 years in the game, meanwhile I cannot remember the last fully RZA produced work that I felt reached his pre-97 standards.