I've only been to Bandon twice, and not since Sheep Ranch opened. Personally, I can't imagine not playing each course every time you go.
After my first visit, I thought that Bandon Dunes was a good course but clearly inferior to the other three.
My opinion of it improved quite a bit after my second visit. I think it has some of the weakest holes at the resort, and I let that color my original opinion excessively. I still do think its weaker holes are weaker than any other course's weak holes, but man the good holes are really good.
Of the four courses (again I haven't played Sheep Ranch) it also beats me up the least. Of any of the four, I'm the most ready to go back out for another round after walking off the 18th at Bandon Dunes.
I love Trails, but after the hike up 16 and invariably trudging through the dunes on 18 after another wayward drive, I'm ready to curl up with a good book at Trails End. Luckily for me that's my favorite dining spot.
I've only played the courses in the winter wind, which means that the 18 at Pacific Dunes is directly into the wind and feels like it's about 900 yards long. I walk off that course ready for a nap.
Old Mac doesn't have quite the same effect, but I still am gasping for a bit of air after the walks up 14 and 15.
I consider the back 9 at Bandon Dunes to be the most playable nine holes at the resort. 10 and 11 are short and fun, 12 is very underrated. When I first played 13 I was asking myself if the rumpled land was enough to make that hole interesting. After another round I decided that it did. 14 has a bunch of ways to play it. I don't like 15, and 17 is OK, but 16 is fantastic. Because of the winter wind, 18 on Bandon Dunes has always been downwind for me so maybe that helps, but I'm always feeling fresh and ready to head right back to one of the first tees after walking off BD 18.