[2020] If you could use any off-hand slot for Necromantic Aegis, quivers and all 1-handed weapons, what would be the strongest most broken choice?

Any good rare one-haned weapon would be the most broken, because of the horrible things that would result from Animated Guardian having the stats of two off-hand weapons at the same time. Depending on the specific stats invloved that could be anything from "crazy op" to "crash the instance", but it would definitely be broken.