SSO shield, I think there's a guide or something on general forums for which SSO quests are quickest to do for multiboxers. YOu also have a small chance to get badges bundled in with your daily rewards.

If you are a 5-boxer you can also kill the first boss in MgT over and over for rep / shards.

All my shamans are using the SSO Rep shield and I'm very happy with it. The merciless sheld may be an upgrade but it's small enough that I will hold off on it until i've upgraded everything else.

And yes, /agree with everyone else, a shield is NECCESSARY in pvp. With a shield and some of the gladiator's gear, you should be pushing 50% damage reduction, which means you will no longer be melee fodder.