well I'm sure there are plenty other rappers with a superior flow it's just that nobody really knows them aka underground artists...
but as far as the mainstream goes:
biggie (his flow fathered a lot of other rappers style)
luda (is there even any purpouse of him existing other than just enjoying his flow?)
jay (from reasonable doubt on, that fast rap isn't for me)
nas (sometimes, from time to time he suffers the talib-syndrome of putting to much syllables in one line)
eminem (come on son, he's ILL with the flows. subject matter is another topic. he has one of the best timings in rap history, hands down)
i'm sure I frogot some. list in no particular order, this also depends on what you like and prefer. you can't really compare the flow of biggie and luda for example, but you may prefer one over another.