Best area I've ridden: Moab.....From the whole enchilada, which is a life-changer like Utah said, anything in the Las Sal's is bona fide awesome, as well as anything off cane creek (amasa back, capt ahab, rockstacker, etc), dead horse, brands, white rim even the trails outa town, slickrock, you could spend a month there and not get tired one the trail network one bit.
Close second is VT for me, there is just too much good stuff, although a LOT, (maybe most) of the good stuff is hush hush, which just makes it more fun to find. KT and millstone are not even close to the goodness thats up there. Everything from rocky nasty steeps to hour-long swoopy flowy bermy fast fun dh and absolutely everything in between.
Pisgah, dupont, bent creek are third...didn't get to explore that much, but there is a lifetime down there, but expect a ton of hike-a-bike
places to hit on my bucket list:
park city and surrounding areas
CB, Durango, Telluride, GJ/Fruita, Gunnison, steamboat, pretty much every CO mtn town has serious serious riding CO trail is on bucketlist, as is CDT and GDMBT, although CDT is pretty much impossible
Tucson, prescott, sedona, flagstaff all look pretty damn amazing AZT if they can get the grand canyon hike-a-bike thing sorted is bucket list
Bend, sun valley, the cascades in and around seattle
whistler, nelson, squamish, revelstoke, etc etc
I have to be honest though, bang-for the buck, proximity to good, if not great riding we have it pretty damn good here in NJ...all of the areas mentioned above with the exception of maybe park city and moab are predominantly one type/style of riding on one large trail system. Its HOURS to drive someplace else. We have loads of great riding within 1.5 hrs. Think about it! Everything from 6mi to wayway to stewart to mooch is all an easy drive to great trails that are well maintained and easy to navigate.
But I do love me some KT time