I remember Green Brook Rd as generally pleasant, but I normally do some combination of Johnston, Valley Dr, Hill Hollow as warmup climbs, then take Valley Rd and Washington Rock Rd to near the top of the Warrenville Rd. Doing Warrenville as my first climb of day could result in injury. It's the real thing. No room for "tacking" or weaving.
Fiddlers Elbow Rd is only the steepest of the interesting climbs on different sides of the "Montana" hill in Warren county. First time climbing it, I definitely recommend approaching it from the bottom with Roxburg Hill Rd --
not riding or driving it down from the top beforehand -- so you can climb it "on sight" and enjoy the surprise of what you encounter around the different bends in the road.
Unlike Warrenville Rd, it gets much less car + truck traffic: a good thing considering how steep it is. If you have any imaginations about climbing Fiddlers to the top without stopping or walking, I recommend first warming up on some of the other "Montana" climbs.
While you're exploring out west . . .
on the Musconetcong ridge in Hunterdon county, Ludlow Station Rd south from Asbury might be as hard (steeper longer?) than Fiddlers -- but I haven't had the chance to try it yet. And it's next to Iron Bridge Rd -- so I'd love to get a report about one or both of those.
Ken