This Thorium Ore farming guide will list the places where I was able to farm the most amount of Thorium in the least amount of time. There are a two types of Thorium Veins you will encounter on these routes, small and rich. It's recommended to have Mining skill 200 before you start farming, because you will get more ores.
I believe Winterspring is the best place for mining Thorium, there is so much Rich Thorium veins in this zone. You could actually just go off the route that I recommend and you will still find Thorium everywhere.
The reason I like Un'Goro is because of the layout of the zone, one big loop around the zone and that's it. It's so easy to farm there.
Silithus is also an excellent place to farm Thorium. There are a bunch of caves filled with bugs, but don't go inside them. You will have to fight mobs and you will get dismounted if you go inside. (You might also get lost in those caves because it's hard to find out.)
IMPORTANT! If you are level 50 and have done the Argus questline, Silithus will be phased and most Thorium Veins will not be there. But, if you talk to Zidormi, she will allow you to switch between phased/low-level and Legion/level-50+ versions of the Silithus. (if you open the map, she will be marked with a white text bubble at northeast)
These are the best zones if you want to farm Thorium Ore. Once you are done mining Thorium, go to my Fel Iron Ore Farming page!