Everything around "Valkenburg a/d Geul" for me ;-) Since it's "back home" for me (and I'm gone for about 10 years now). So always nice to get "back home". And the views are just great. I never bike there for the sake of "racing" but to make some relaxed miles and enjoy the views/environment... plus as a bonus I get to spend time with my parents afterwards (since I only see them about 8 times a year)
BTW: Perfect for retro riding, it's got the most hills of all the Netherlands but still you can get away with not having suspension