or Pit of Saron. Just make sure you don't go more than a few hours. Dungeon farming is very risky.
Naxx takes longer per run. You have to manually clear up to the mobs on 25-man mode, then go out and switch it to 10-man before you can start farming. And then you have to run to the room each time, which takes too long IMO. But it's probably safer, idk.