In which locations can I mine the most iron ore?

The best place by far is in Brisband wildlands at Gallowfields Waypoint, here is the wp link to it [&BGMAAAA=]

First of all, there is a rich iron node just by the wp, and a lot more nodes are scattered in the area. You can easily get 20-30 iron ores from one run around a small area.

Just beware to jump down the right place, as the rich ore is down in a cliff, and falling all the way down will kill you.

I would go to the Iron Marches. The maps in Ascalon that are Charr based have a lot of iron.

If you go to this site gw2timer.com/?page=Map&section=Resource and select iron and "show possible nodes" you will get a good map of where to start. This map is zoomable.

Guild Gathering Bonus will give you %15 bonus chance to harvest more, so will Guild Hero Banners.