Hobbies And Interests
Hellfire Peninsula
Hellfire Peninsula is the first Outland zone you would encounter as a leveling player and has the highest concentration of Fel Iron deposits. The deposits are scattered all over the zone, although they're mostly clustered around geographical features such as mountains and cliffs. Furthermore, no ore deposits of any other type are present, making traveling around this zone in loops the simplest way of collecting large amounts of Fel Iron Ore.
Zangarmarsh
At first glance, Zangarmarsh may seem like a good zone to gather Fel Iron Ore in due to the location of the deposits. It's true that the deposits are found mostly around the edges of the zone and therefore allow for more efficient travel, since you have to cover only the edges of the zone rather than the entirety of it. However, the number of Fel Iron deposits in Zangarmarsh is less than half than that in Hellfire Peninsula, and several of them are clustered in cave systems, requiring you to dismount and proceed on foot. Additionally, Zangarmarsh also has a handful of Adamantite deposits, which will be useless to you if you're interested only in Fel Iron or if your mining skill isn't high enough to gather Adamantite.
Shadowmoon Valley
Although it only has a third of the amount of deposits in Hellfire Peninsula, Shadowmoon Valley can nevertheless be a fairly efficient location to gather Fel Iron Ore if there's heavy competition for deposits in Hellfire Peninsula, assuming your character is a high enough level to deal with the enemies in the zone. The deposits are fairly clustered and in obvious locations, such as near mountains, and there are no cave systems you would have to navigate on foot. This zone is also ideal if you're interested in both Fel Iron Ore and Adamantite Ore, since on average you'll encounter the same amount of both types of deposit during a single loop of the zone.
Other Zones
All zones in the Outland continent have some amount of Fel Iron deposits. However, because zones were designed with the expectation that higher-level players would need more Adamantite than they would need Fel Iron, in higher level zones you're likely to find a much higher amount of Adamantite deposits than of Fel Iron deposits. This holds especially true in the Nagrand, Blade's Edge Mountains and Netherstorm zones, while the Terokkar Forest zone is more balanced.