Pat:
I think Machrihanish at 6462 yds is very good (and that's with a bunch of new tees; the everyday yellow tees still play at under 6,000 -- 5971).
Fraserburgh, subject of a recent profile by Ran, is 6308 from the tips and regular yellow-tee play is right around 6,000 yds. Maybe not quite as good as Machrihanish, but I'd put both on a top-20 list in Scotland for sub-6,500 yds courses. Boat of Garten at 5876 yds is a terrific challenge; if you re-located that course to Minnesota or Wisconsin, people would rave about it over here. A probable top-20 in Scotland as well for the sub-6500 yd category.
Here in Wisconsin, I think Maple Bluff (6,400 tips), Blackhawk (6175 tips), and Spring Valley (6451 tips) are all really solid courses. And the criminally unknown 9-hole Country Club Estates in Walworth County near Lake Geneva, played twice, is a lot of fun at 6,022 yards.