Well it's not as cool Central Park nor is it as earthy as Golden Gate Park but for a quick city MTB ride Frick park in Pittsburgh can't be beat. I usually start at the Baseball field and pick up the Iron Gate trail which is to the right of right field. I have never had a problem with hikers, dog walkers...
Great way to escape into a woodland without heading far out of the city. Although the occasional sewage smell is a reminder that you are in fact in a city-like park.