That rather depends on your level.
One of the best zones I know of for iron is the Badlands. Some of the iron spawns more towards the middle of the zone, but if you just ride along the mountainous edges you can gather a good amount of iron fairly quickly, assuming someone else hasn't beaten you to it.
There is also a fair amount of mithril in that zone. The areas with the black whelps are particularly rich in mithril.