Your lore should be specific to what you are trying to accomplish. Shadow is a good one to use against many opponents. However, light and life are not to be frowned upon either.
Light works very well against HE and Bretts. HE because you can cancel that horrendous reroll they get every turn (ASF + higher Initiative). Bretts work best by getting the charge (+2 str on their lances plus that 2+ armos save). You can use Light to potentially get charges using Birona's Time Warp (double movement up to 10, extra attacks, and ASF). If you manage to get that off and get speed of light, you are rerolling you misses on those rounds of combat.
Life is a good general lore to give your slann. You can protect the slann from miscasts without taking cupped hands of the old ones. I always take the cupped hands because I have a pretty reliable recordd of rolling 1s on the throne of vines save. Also, the lore attribute of life will help you heal your characters and multi wound models within 12 inches of the slann. Lastly, the buffs that you can cast with life are very good. You will be looking at T8 Temple Guard, saurus, and even T6 skinks. If you can get the throne of vines cast.
I do not really see any good use of the lore of Fire. A couple of magic missles. Nothing else though.
Lore of Death can be fun and useful. I do not usually cast the purple sun spell. It usually ends up killing my own troops. The sniping spells are very useful though.
Lore of Beasts is good for buffing characters, but I think it is wasted on our army.
Lore of Heavens is good, but if I am going to use it, I will just take a priest.
Lore of metal is also situational as it uses your targets armor save to wound. If you use it against Bretts, it will devestate them. Use it against armies with little or no armor save and it is a waste.