Very difficult to compare.
Biggie has the best flow ever without question imo, but lyrically no one can see Nas and he's more versatile.
Also their styles are very different, Nas is more of a poet and Biggie a straight street rapper for the most part. Nas raps in the third person often, Biggie is almost always in the first person.
Also Biggie only released two albums whereas Nas has literally been rapping for three decades now so we have a much larger body of work to draw from.
Biggie is defintely 2 or 3 on my list, I would never argue with anyone putting him number 1.
There are just so many factors to consider when picking greatest rapper and everyone has their own preferences.