Ahh zerk... Well zerker is a damage dealer first of all, but again it's all up to your preference. There are resonance that increase your rate of critical hits or maybe you would want to choose resonance that increase the damage of your critical hits. There are also resonance that add elemental damage, such as Raging Flame and Ice Vortex and Speeding Wind. I've also heard there are other resonance stones not available in shops that have other powers. In the end, it's all up for you to decide on what resonance stone you believe to put in your weapon/armor.