Actually, I think Crumb is over-rated. I would replace him with Peter Mennin, and if I had to single out one work, the piano concerto.
For "unknown", "American", and "piano concerto", my own choice would be the Giannini that was recorded on Naxos a few years back. That's a stunner of a work if ever I knew one.
As for this list...well, I'll just say that most of the composers listed aren't to my taste, really, and leave it at that. I think my own list of ten would be vastly different than Mr. Slatkin's.