http://incursions.nexsoft.de/?mode=guide&id=6

Mod Stacking effectivess: Mod1 = 100%, Mod2 = 87%, Mod3 = 57%, Mod4 = 28%, Mod5 = 11%, Mod6 = 3%
Damage type to protect against:

Angel - Explosive & Kinetic
Amarr Navy - EM & Thermal
Blood - EM & Thermal
Caldary navy - Kinetic & Thermal
Gallente - Kinetic & Thermal
Guristas - Kinetic & Thermal
Khanid - EM & Thermal
Mercenary - All
Mordus - Kinetic & Thermal
Sansha - EM & Thermal
Serpentis - Kinetic & Thermal

Ideal Damage type against enemy:

Guristas - Kinetic
Serpentis - Thermal
Mordus - Thermal & Kinetic
Blood - Thermal
Sansha - EM
Angel - Explosive
Mercenary - Thermal

Amarr: God says I'm better than you, so just stop fighting back.
Caldari: I don't need a fast ship, I can hit you from here.
Minmatar: Hold on, as soon as I find something heavy, I'll kill you with it.
Gallente: Just wait 'til I get in close range...