Pretty sure it's the warboard. Though, + 2 AC for everyone sounds great to me. It's technically the best shield in the game (2+2) and let's you bane the odd enemy. At cost of a bonus action instead of action. Up to 3x/day
Yeah, I was hoping he didn't mean The Absolute Warboard since that the best shield in the game - especially if your entire party has the Mark. I think maybe he doesn't understand how it works.
You don't even have to keep wearing it, just use its ability on the rest of your crew (No Pets/Summons) and then equip something else. Its a free +2 AC each for your 3 other teammates and it can't be dispelled.
And yeah you can also use it as a Mass Bane. LOL