Most usefull +10 in shields

For melee enemies it does depend on the weapon they use. But if you have a +10 vs creature mod then all armor respecting dmg from these creatures gets reduced by that +10 armor (so slashing, piercing, all 4 elemental dmges, dark dmg from wand or staff attacks, etc.)

Useful +10 creature mods can be:
1. vs demons for almost the entire realm of torment and the entire Domain of Anguish
2. vs undead for all of Shards of Orr or Cathedral of flames for example
3. vs dwarves for most of the southern shiverpeaks and dungeons like vloxen excavations
4. vs plants in the echovald forest and urgoz warren, although I recommend +10 vs piercing in urgoz warren for a wider coverage of enemies
5. vs charr in ebon area eotn and old ascalon
(6.) vs ogres in the norn area against Jotun but much of their dmg is armor ignoring and the mod is very niche

Keep in mind this is all just for fun and if you actually want to survive better you bring an st

it really comes down to what you are facing. armor mods are most useful against things that accumulate the most damage to you. for example warriors are pretty well armored against physical damage already, so a elemental mod often reduces overall damage more than an additional physical mod. also consider armor calculation (check wiki or google it): inherent armor mods are treated as bonus armor and thus often are neglected in regular gameplay while you are under effect of another armor buff. this means a insc shield +10fire won't behave as the same one in os. but depending on your build, team and buffs they might be equal.

on top in regions with just one creature type or primarily one the creature mods can come in handy. I think HeavensDevil covered the important ones. the remaining being dragon, giant, tengu, troll, which just do not really occur in greater scope whilst ofc there exist some farming locations where those are relevant - and skeleton, which mainly is an inferior undead mod, but in UW/FoW tge skeletons seem to belong to other armies than undead and as such the skeleton mods come into play.

in southern shiverpeaks for example you might bring a cold mod on an insc shield rather than a dwarf mod on an os one, if you decide to bring armor increasing skills in your bar or allies that'd supply such to you. but here it gets a little too much into detail, it wouldn't affect gameplay much. after all 10armor is just a reduction of that type of damage by ~16%...

something else to consider: lightning damage spells often have 25% armor penetration, so a lightning mod is in tendency weaker than other elements. physical mods come on top of your other armor privileges as a warrior, so for a fixed damage of 100 against a 60ar target that packet will get fewer reduction if it is physical and you use the proper physical armor mod than compared to if it were a elemental and you used the proper armor mod. so it comes down to the distribution of phys/elemental damage - if you take more from the one take the armor mod against that. where you already take small damage packets a damage reduction is more effective (also it is not penalized by armor penetration), so a -2we does never harm on top of a armor mod - but that again is only possible on os shields, where the armor mods are inferior compared to insc... so just ideally bring a dualreduction shield

+10 troll is mandatory in both shiver peaks and kamadan