I tried ungoro and silithis on my lv 68 alt while farming 240 thorium ores. I found going along the edges of silithius by far the best because there were more high yields and some ungoro nods were difficult and time consuming to reach without a flying mount.
I think some of the nodes in Silithus are underground though, which is a pain. Id say winterspring. I think a lot of people kind of forget it's there. Either way though, try doing a /who for all 3 places and go where there are less people
Hands down Silithus has a lot of Thorium (and consequently Arcane Crystals).
Have a look at the following map:
The Teal dots are Thorium veins - Black centre = normal vein, White centre = rich vein. Purple dots are Truesilver. Squares are various herbs (Silversausage, Dreamfoil, Sungrass etc.) The triangles are Silithyst deposits.