Well, there is that witch... But generally low level mobs become completely obsolete for XP after a while, so a farm wouldn't be useful for long. What I've generally seen is just running some high level area, often a resettable instance, which is very efficient to run for XP. So not necessarily the strongest enemies, but the ones you can kill the fastest in the highest amounts while still getting decent XP from them.
On the other hand, item drops tend to be much more valuable than XP, since you hit the level cap much faster than getting the best gear, and those tend to be what you get from raids and other end-game challenges. But for XP, that makes it weird when it always takes so long time for all characters to reach the level cap (or the practical cap for the released content, which may not be the technical level cap).