Sandpiper GC near Santa Barbara, Ca.
W.F. Bell design that I think is better than Torrey South, and I like Torrey South alot. It's better than the overrated Trump course in Palos Verdes or either of the Half Moon Bay courses in my opinion.
It's sister course La Purisima is pretty good too and doesn't get much attention.
Not sure I agree with either one of these. Those in love with the ocean views love Sandpiper and probably think it is better than it is. Those annoyed with the first group probably underrate it. I think it is pleasant golf and has a few of very good holes (10, 13, 14) but not sure I'd call it it underrated. I played there a lot in grad school and do always enjoy playing there though. I agree that it is better than Torrey South, but would say that Torrey South is more overrated than Sandpiper is underrated. I am not a driving range guy, but I love Sandpiper's. Very relaxing.
As for La Purisma I enjoy playing there as well, but more for the incredibly peaceful setting than the the quality of the architecture. The course can be extremely difficult and is fun, and is always sort of a cultish favorite in California, and it my mind that is probably about how I feel about it.
Obviously just my opinion to take or leave as you prefer.
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As for quality designs that are completely under the radar except for a few in this crowd, I'd say that the Santa Anita Golf Course, a Los Angeles County course that does as many as 110,000 rounds a year, is underrated and really an impressive design.
But this is of course on a different level that Prestwick and some of the others mentioned.