At level 18 I am given a choice as to rather to increase poison, disease, heat, cold, or mental resistance. Which is the most commonly used attack spell type that mobs use?
good news is that you will get the choice again periodically throughout the life of your character.
easiest thing to do is to check your current resistances (p key) and improve whichever one is your current lowest. If more than one is lowest, flip a coin!