1. "There was the one on Stretch and Bobbito over..." In response to Reply # 0
Large Professor's "Gotta Get Over" remix. I don't think any of it was actual "freestyle" beyond telling Stretch, "Ya fuckin up, kid" after the record skips. I don't remember hearing the rhymes he kicked on that showing up anywhere else though. But it's been a while since I heard it.
9. "There is a story of Nas freestyling in BIG's crib for hours" In response to Reply # 8
It got to a point where BIG said "I aint fucking with dude". The word was you could tell Nas was freestyling because he was incorporating stuff from the room they were in.
That said I've never found him to be a compelling freestyler, but an amazing writer... perhaps the best in hip hop.