Andrew - I think #18 at Dunbar is a superb hole, with the rolling fairway, the interesting green complex, and of course the OB wall on the right (somehow, internal out-of-bounds seems much less annoying when a stone wall is involved!) to ask a delicate strategic question, esp. from the tee. #15 and #17 are good enough holes, but I think #18 is a better long par 4 than those are short par 4s.
By the way, Longniddry is a truly unique course - half-links, half-parkland, with a seaside feel that reminds me for some reason of Cape Cod more than Scotland. Lots of really good holes, too. Doak gives it a "5" rating in his book, which is about right, but it's perhaps a notch or two higher if you're looking for something different...
Cheers,
Darren