The "stock" answer is which ever course you are playing.
My preference is the Old Course because it has the best green complexes, excellent movement throughout the property, and great variety of holes. (I understand that some may not like Fazio's "tuck and roll" when he re-built the bunkers but the putting surfaces were not altered.)
Weyhill certainly has a WOW factor and when I bring guests to play all of the courses, this is usually their favorite. I think this is the one of the best inland/parkland sites in the US. The Saucon Creek meanders throughout the course and it is used with great effect without being too penal. When you get to Hole 14, you must carry a huge abandoned quarry. Hole 15 returns in the opposite direction, the quarry is on the left as you approach a severely elevated green. (I can't wait until tree removal occurs and this world class feature is no longer needlessly obscured.) The last seven holes are superb. An excellent blend of short and long par fours, a beautiful dropdown par three, and two well-conceived par fives.
The Grace Course is also excellent and many single digit players will say this is their favorite. Interesting architectural note: when William Gordon built the par 3 fifth hole, he corresponded with a surveyor in St. Andrews to recreate an Eden hole!
Obscure architectural note: When I joined SVCC about 18 years ago, I wrote to David Gordon and asked: "Since your father had all of that land to work with, why didn't he place his Eden with the Saucon Valley Creek behind the green to more closely emulate The Old Course?" He wrote back and said: "Dad did not believe in hazards behind greens, he felt going long was penalty enough."