The kind that doesn't result in a lost ball or prevent any attempt at recovery shot. So, I don't like difficulty that involves lots of water, OB, and long rough.
I'm down for difficult that involves firm and fast, undulating greens, necessitating angles, centerline bunkers that result in chip outs (can still try to hit the green and one putt for par).
Wind is the ultimate in difficult, especially combined with firm and fast, but it can't be relied upon.
Pinehurst #2 is the kind of difficult I like. I'm basically never going to lose a ball. But, it is very hard to get good birdie looks and very easy to make bogey because of the difficult of getting up and down. As a plus handicap since I was 16, I never managed to shoot under par there from any set of tees in almost 150 rounds.