I have a 5th level Paladin with Ankheg armor. He has a -5 AC I give most of the magic items to the rest of my party I have one NPC that is at -2 two at -1 my thief (Imoan) is at 2 but she uses a bow. and another at 0. I think the best AC is a well rounded one with 1 or 2 people as far below zero as you can get. what helps this though is HP lots and lots of HP.