I wouldn't call them rare. They wouldn't even rare then. Can't speak for the whole list, but half of them are officially available. The Basement Tapes came out 2 months after this article was published. Dylan's Judas concert has been around for 20 years. Live At Leeds is official so I don't know what the hell's its doing on there. Prince's black album, as is mentioned, was also officially released. Beach Boys Smile is out there in nearly complete quality.
Weird list. I'm not good with bootlegs, or rare ones, so its tough for me to judge, but any list that has half stuff thats already out in official quality I'd barely call the "rarest and best". Yes, maybe they are the most famous bootlegs, but I'm sure there are legitimately rare and incredible bootlegs out there and this lists seems to lack any of them.
I don't know about how rare they are but for my money two of the best bootlegs are Little Feat Electrif Lycanthrope & the Rolling Stones Bedspring Symphony