What is the "best" tank for multiboxing.

[Rin]

I'd argue that for a tank + 4 shaman team, DKs bring far more synergy than a paladin - the 13% spell damage bonus is the most synergy you're ever going to get for a group that has 4 offensive casters. For groups without 4 shamans, sure, a paladin probably provides more synergy, but you can't really argue that anything a paladin provides to a group of 4 shamans comes anywhere close to +13% spell damage.

This is correct. If you're going with 4 shamans, then you should go for the +13% damage increase from the unholy DK. If you're going with 3 shamans/druid .. then a Paladin will get you more mileage (Kings), since the Druid will bring the +13% damage enhancer and a handful of other useful buffs (thorns, mark of the wild, etc.).

[Bigfish]

You know, I think BigFish tried using a Ret. Pally for an offtank while 10boxing.. didn't work out so well if I recall correctly.

It was Ret pally maintanking, and no, it didn't work out so well. It works on a level if nothing hits you for any significant damage, but you'd still like some mana to tank the stuff, but the added squishiness SUCKS.