Fire cuz most dmg, easy as that^^
And another nice thing is the increased mag defense which is important for a str built.
Lightning would be second, increased moving speed and so on...
Cold is a waste of sp in my opinion, it got the phys dmg increase buffs but as a str you wont have problems with that anyways. And I figured that its really easy to become completely immune against cold, already with 60% frost reduce and the fire shield series its impossible to get frozen by someone your lvl even with the 100% freezingprobability skills.
Server: Babel
LvL: 7x Hybrid int Sword
Weapon: Hellfire Devil Sword+5 / Divine Iranggjingun Pike +5
Equip: Deva Protector +4; 1200HP; 100% Immunity
Mastery: Fire, Lightning, Bicheon, Heuksal
Guild: Taizong (lvl 5)
Job: Hunter/Trader
Status: Fully farmed/ retired since march