When you put the question that way, it's very hard to distinguish between the courses with the most SEVERE greens and the most INTERESTING greens.
The first lesson is that NONE of the top ten courses have anything less than difficult greens. The most innocuous greens of the bunch are Pebble Beach's, but they are mostly 3500 square feet with a fair amount of tilt, so they are anything but easy. Shinnecock's don't stand out as severe when you first play it, either; but if any of you remember Retief Goosen putting off the first green on Sunday of the Open, you will give up on that line of questioning. Only a few of the greens at Cypress Point look obviously severe (7, 8, 9), but a lot of them have enough back-to-front tilt that can make putting off the green a distinct possibility.
At the other end of the scale, Oakmont, Augusta, Pine Valley and National are the most difficult of the ten.