This is hardly a definite answer, but: I suspect it would be lightning (overall)So which element yeilds the most damage. (Best passive resistance to add the most into for damage bonus). Fire, Cold, or Lightning?
... the synergies all provide the same amount for each respective element, so that has absolutely no difference. My opinion of Lightning as better damage is entirely based upon the fact that most/all monsters have breakable lightning immunities (although fire isn't too far behind, you're still more likely to meet an unbreakable FI than LI)... with your conviction aura active, you'll break the most immunities in Lightning, and therefore your Resist Lightning synergy will kick in more often.
Also, IIRC there are more FI and CI monsters in the game than there are LI.
However, I doubt the overall difference from various damage types is significant. Even if you get an extra 100 damage out of lightning, that's still likely to be a small percentage of your overall damage. What you might want to consider first is the passive boost to maximum resistances that you gain by placing 'hard' skillpoints into the Resist <element> auras... I think this will have a bigger affect on your Avenger's gameplay experience.