For console, since you don't yet have One Tamriel, I would definitely do nMA, or, if you have friends to do it with, nDSA (if you don't care about the XP and/or gear, you can even scale it down, and you'll still get loot since the loot comes in chests, but since there's a cooldown of around 3 minutes, there's really no point in going too fast).
On PC, though, I prefer farming a normal dungeon since the loot is now a lot more interesting thanks to One Tamriel. Pick one with a gear set that you're interested in and kill two birds with one stone. I had held off on farming Burning Spellweave specifically so that I could do nCoA1 runs to get both skulls and Spellweave.
I still have a few ...starkerealm wrote: »You should be asking, "What are Plunder Skulls?" Because the answer to that is, "it's already seven months too late to get one."