It might be Ice Cube
For a fall off to be really big, you had to be in elite group of MC's at some point of your career. And Cube was in early 90s. But soon as his Hollywood paychecks got big, he stopped making good music and never came back. With some of his songs being good once in a while, but albums being absolutely inferior to his early 90s work. He beat NWA and then became a parody of NWA for many years. That's a definition of a fall off.
Prodigy did have a big fall off as well, but at least he tried redeeming himself later on. Cube just been rapping in Dr. Seuss rhymes for about 20 years already.