Jim,
No matter which courses you play, you'll have a great time. As to which to skip, it's sort of like asking "which Charlie's Angel should I date?" You can't wrong.
That said, unless your group is all hard core architecture junkies, DO NOT skip Sheep Ranch under any circumstances! Everyone will say it was the most spectacular place they've ever played. The opening drive is a gentle handshake, then when you crest the ridge -- BOOM! -- the entirety of the resort unfolds before you to your left, with the green perched dead ahead above the crashing waves of the Pacific.
The best course is certainly Pac Dunes, so that too would be a must play in my book. That leaves Bandon Dunes, Old Mac or Trails for the one to skip. Again, I'd think about your group. Bandon Trails is my third favorite course, but it is certainly the most difficult walk (remember, all play is walking only, by your last round you'll be tired). Old Mac is probably the "easiest," with tons of quirk. My choice to skip would be Bandon Dunes, the original course. Don't get me wrong, it's great, but it doesn't offer anything that the others don't. The same can't be said by excluding any of the other courses.
Have a great time, and please report back and let us know what you decide!