Shadowmoon Valley
Why are we here: Instant respawn cloth wearing warlocks.
Who drops it: Shadow Council Warlocks
Why are we faming these: They respawn almost instantly and drop fel armaments, Mark of sergeras, and a small ton of netherweave, all in addition to normal mob drops.
Where to farm: coords 23,38
What else is in it for me: you need more? fel armaments and marks not enough? OK, how about they also drop Fel Strength Elixir and Major Shadow Protection Potion recipes!
notes: By instant respawn I mean the number in the area has a set minimum. this means that if enough die there will be more spawned instantly. the upshoot of that is, if you're sharing farming space with someone else they actually speed up your kills. I farmed for 3 hours without moving one night.
Watch out for the roaming elite, he's a pain.