So many people recommending stone armor when it has no slow resistance and no cold defense. It’s super annoying. I think it also has no endurance drain resistance and no ranged def, so sappers are an insta gg.
Willpower has the some of the same problems. No ranged def and no slow resistance gives the Tsoo siphon speed spam and sappers free reign.
I like Bio armor because I can defend against and plan for almost anything. Even against defense debuffs I still have S/L resistance and absorb, especially in defensive mode. You should also slot for energy resistance with Bio because energy damage is common and comes with -def fairly often too.
Fire, cold, dark and psi rarely come with -def so your defenses in those areas should be sufficient. Even in arachnos maps with the psi debuff ‘scramble thoughts’ most of the actual damage coming at you is S/L and energy. And just kill the psi enemies first.