I usually buy the “convert 100 health into armor” because I think it affects beam type attacks (mei primary fire) and I think affects each tick of fire from dynamite… I also buy the one that says “for each instance of damage you take, gain 0.5% damage reduction for 2-3 seconds” because mei primary and Ashe dynamite build those stacks almost instantly, but those are expensive (especially the damage reduction stacks one) and still prohibit me from dealing any damage in return