This is an incredibly subjective question depending on the influence of what type of rap you listen too....
I have had that Biggie n Pac freestyle for a while, and it is cool. But out of everyone, I would have to say Canibus is the best freestyler. His freestyles are 1000000x better than any song he has ever put out, I have a track of him at home (at work right now) that he goes on for a bout 7 minutes straight... and it is just great.
Eminem is an okay freestyler, I don't know why people think he is the best.... its good, but its not great. They all seem somewhat short and really are just words rhyming instead of saying a lil' message or sumfin.
Surprisingly, the Biggie n Puffy freestyle I have is pretty damn good. Method Man and Redman have somem good freestyles too, but they still aren't Canibus' style. Now... Lil' Flip is good if you like the H-Town ghetto style with a DJ Screw beat. They don't call him the freestyle king for nothing (and I am sure not many people out of H-Town ever know who he is ... except he has a new video out)
People from the West will probably claim 2pac
People from the East may claim Canibus but probably Eminem
People from the South will most likely claim Lil' Flip
Spac3d