If you play so many characters, I would suggest starting with runes with universal application: MP Regen, Cooldown Reduction, Movement Speed, $/10, etc.
If you are limited to only playing mage-types, try Magic Resistance Penetration runes, they will give you more bang for your buck than AP runes in terms of damaging spells (for casters who focus more on heals and shields, AP are still better). For most physical champs, Armor Penetration is likewise very good. Crit chance is good for those who are based on criticals, and Crit Damage is good for those who get a high crit chance normally (ashe, tryndamere, etc). Straight damage again isn't the best, as it does not scale at all.