I loved the set of greens at Merion. They come in all shapes and sizes, and they test every type of shot around the green. Furthermore, the greens ask several different things of players from the fairway. This was the first course I have seen with a truly great set of greens.
The Ross greens at Oak Hill (West) and Teugega are phenomenal. I would put the West Course's greens slightly ahead of Teugega.
I thought Prestwick had a great variety of greensites. Some are rippling (1, 2, 4, 12). Others have only one dominant slope in them (6-10). The two greens by the clubhouse are downright baffling (14 and 18). Finally, Prestwick has some of the most beautiful, outrageous, and crumpled greens in links golf. The greens at 3, 13, and 15-17 are endlessly interesting. When I played Prestwick, I spent about 5-10 minutes around the greens at 15-17, trying all different types of shots with various clubs. Tons of fun.
Simon, I thought North Berwick has a solid set of greens from start to finish. The 12th might be the best of the bunch, and the 3rd is one of the most unique greens I've seen. However, A few of the greens just don't stick out in my memory, which brings it down a bit in my book.
With all of that being said, my absolute favorite set of greens that I have seen or played has to be DEAL.