Actually the tank that gets the least value out of DM:I is a guardian druid, because they can actually hit armor cap with or without it. Most value is probably gained by Monks and DKs.
Whether it is the "best" depends on a lot of things, mainly what fight are we talking about and what externals do you have access to. I wouldn't run DM:I if I was tanking the spirit realm on Desolate Host, but on other fights it's great. DM:I and Writhing Heart are the best overall damage reduction trinkets, where DM:I only works on physical damage and WHoD only works on things you are actually hitting. Recompiled Guardian Module looks like hot garbage tbh, you are probably better off equipping cake from Trilliax and planting it in melee. The numbers on that thing look well shit.
Engine depends on you actually getting the orbs to prolong it, so I think it is pretty weak on tanks, you are probably better off using Memento or something like it.