Presumably you plan to wear your armor of the fallen in undead or fiend strata (places you will encounter a lot of shadow attacks), so think about what other types of damage you are likely to encounter. Undead often go with slime in the Scarlet Fortress levels, so a piercing shield would complement the armor well. My favorite piercing shield is the barbarous thorn shield, but ironmight plate is perfectly viable and won't break as quickly if you get smacked by a lumber. A shadow shield, like skelly, could be good, too, to really make yourself strong against shadow attacks. If you're worried about fire, grey owlite is great against fire attacks and would add some elemental to your setup.

There really isn't a bad choice among shields,