First of all, a knock off lens is a lens that is built to look like another lens. Like a fake of sorts. Ever been offered to buy a pair of 'real' Oakley sunglasses from a sketchy street vendor?
Second of all. The lenses your referring to are more appropriately third party lenses, which is also subjective to the Camera body your buying. Many people consider Sigma lenses to be third party, but when you have a Sigma camera body, and buy a sigma lens, then its first party.
As far as third party lenses go, I find Sigma, and Tamron being the top two optically. However, if you take a gander at some of the M42 mount lenses, or some older hasselblad lenses, you can get them cheap, and when I say cheap i mean CHEAP, and they usually outpreform their modern counterpart.
For example. The SMC Takumar 85mm f1.8 vs the Canon 85mm f1.8
The SMC takumar 85mm f1.8 is an INCREDIBLE lens. Depending on the condition of the lens it can be anywhere from $200-$400.
The Canon 85mm f1.8 is a pretty good lens for what it is. It focuses pretty quickly, acceptably sharp, Bokeh of reasonable quality. They are usually $350-$400+ New
So whats the difference in these lenses? The SMC takumar being an old m42 mount does not have auto focus, but if its for portraits, thats not necesarily too important. However at f1.8 the Takumar is as sharp as the Canon f1.8 is at f/4. Thats just the beginning, I would suggest you do more research.
Thats just one example as well, I have a Carl Zeiss 150 mm f/4 Sonnar that I got for $150 and is a beautiful beautiful lens.
Anyway, rant over. Good luck