If the situation is one where you simply do not have enough time to play all the courses, indeed a conundrum has been created.
There is no correct answer here, only opinions. Thus, I offer the following:
1. You play Bandon Dunes because it's the original. If not for the success of Bandon Dunes, the rest of the resort probably is not what it is today. You play Bandon Dunes because it offers some nice variety to the other courses. Bunkers not the craggy, mean type. It's a little bit easier walk. And the 16th hole is my favorite place to be on the entire property. Utterly beautiful. You can't go to the Bandon Dunes golf resort and NOT get yourself out there on that point where the 16th green/17th tee resides.
2. You play Pacific Dunes because it's the highest rated, and for the most part the consensus best course there.
3. And I think you play Bandon Trails because it's fantastic! If golf is about the walk and about discovery, Bandon Trails is the most unique course on the property. It might not occupy the storied coastline property, but nor does Pine Valley rest upon a coastline! Trails has oodles of signature holes, can be extremely challenging. I just can't imagine going all the way there are not playing it.
4. For me, Old Macdonald is the omission to make. You'll see 75% of the course from various vantage points while playing Pacific Dunes. You'll have the visual of what the land looks like, where it is located and the challenges presented. I don't say play Trails and don't play Old Mac because one course is necessarily "better" than the other. I say what I say to take in as much of the Bandon Dunes Golf Resort experience as possible WHILE LEAVING OUT one of the courses.
5. And you play Sheep Ranch, obviously. It has more holes on the coast than any of the other courses. It's the newest and who doesn't want to say they played the new course at BDGR. It's BUNKERLESS, so you have some serious variety there. Probably the absolute best vantage points of the entire facility. I just can't imagine leaving it out.
So, there you have it, I'd omit Old Mac if forced to leave one course off the playlist.