If you're going for just bones, the shadow wisps or horde minions are good. Zombies are okay if you want to get other stuff (like trying to farm that Impaler guy in the Haven ruins to get the rare bone armor pieces).
If you need Spined leather AND bones, go to Destard, to the passage towards the lower levels, and farm the giant serpents for both bones and spined. I don't have the patience to kill the other stuff and get NOTHING from half of them, so I tend to go for the Serpents, so at worst I get the leather.
If you're a miner, and have a fire beetle, some people swear by the ant lion in the solen tunnels. You get the potential of multiple bone piles, possible bone containers (too bad they can't be cut up - come back for them if you want them), plus an amount of ore (dull copper to Bronze). Pile the ores up by color, and smelt them before you leave.