To me, great par 5's come in two distinct categories. The first, a hole that truly is a 3 shot hole yet requires thoughtfulness on all three shots. It could be reached in two by the very longest hitters, but it's usually best to approach the target with a more precise club and play for birdie that way. The 13th at Rustic, the 13th at Pasatiempo, the 5th at CPC, the 14th at LACC North are some examples that come to mind.
The second is the clear risk/reward factor. The hole is reachable in two, but it must be executed flawlessly. 9 at Pasatiempo, 6 at CPC, 6 at Pebble, 10 at MPCC Shore are examples that come to mind.