If you want to have your head clear when evaluating portraits, stay away from ones where it's either a celebrity in it or one behind camera. I say this as I assume the question is about pure visual impact / value that makes it stand out, not it costs now and will appreciate to, neither a determinant of what it actually looks like.
Also, Leibovitz was mentioned here, I'd put here at a bottom of the pack regardless. Andy Warhol ??? That is pretty much what I am alluding to at the start of this post.
I do agree that taking a look at Jane Brown might help. Certainly far above Leibovitz IMO.
Better yet, scan web and YT. Thousands of outstanding portraits are taken worldwide every day, many get posted on the net daily as well. The whole spectrum of worst to best gets filled more and more and gets ever more interesting.
Key is to view it without having any clue who is in it or who is behind it.