Beasts of Chaos - I usually pick fire for MM, but as they have trouble with LD Death can be great (-3 LD + terror on krox / CoR)
Bretonnians - Metal. Tons of armor which is great for the metal lore, and Burning Iron one-shot snipe BSBs or other heroes with bane head.
Chaos Dwarves - Lots of armor and war machines so I guess metal would do well here. Fire is good all-around but iirc the CDs has some immune to fire items (which i guess would punish the metal lore just as much).
Dark Elves - Sort of depends on what style of army he plays. If he uses Ring of Hotek I usually pick lore of metal, then drain his DDs on stuff out of the rings range then bane head one-shit him, using 2 dice. Fire works too but you cant wall of fire snipe the RoH guy since the casting value is too high.
Dogs of War - War machines and knights are pretty common here, metal or fire would be my pcik but Im yet to meat a DoW player.
Daemons of Chaos - Lore of Light is pretty decent here, but if you play closed lists I wouldnt take the chance in case he has the banner. Metal & Banehead is again an awesome combo for sniping those pesky 0+ Jugger heralds, but as spirit of the forge and Law of Gold is fairly useless you are left with only a few decent spells. Fire is great aswell, lots of damage spells. Shadows offer some good mobility and Pit is VERY useful against greater deamons (even against KoS and BT 1/6 chance to 1-shot them isnt bad).
Dwarfs - Metal, same motivation as CD.
Empire - Low LD armys so Death and Fire are good. Metal is decent, especially if he has a inner circle unit with characters.
High Elves - Fire and Metal isnt the best choice for once, as some units are immune to fire. Heavens do alright if you want to keep characters at a distance (comet wrecks them). Also has some useful spells to boost your shooting and combat, like Portent or Second Sign.
Lizardmen - Probably depends on the army again, one-shotting the 0+ mounted BSB is pretty neat, and saurus, temple guard and CoR gets destroyed by SoTF. Shadow is pretty nice too, sending your scar veterans flying into annoying skink units or dropping the slann/carnosaur/anything down a pit of shades.
Ogre Kingdoms - I have no idea tbh, havn't played them with a magic build. I guess fire? They dont have a lot of armor and are vurnarable to any form of ranged S4+ attacks.
Orcs & Goblins - Pretty bad LD but not terrible, Death, Fire and Metal is against solid choices. The lack of good cav or extreme fighting characters makes Fire my first choice ahead of metal. If you face the wyvern build you can take shadow for Pit or beast for Beast Cowers, but personally the wyvern is managable without those spells and I the rest of both lores are pretty suboptimal.
Skaven - Haven't met the new skaven so can't really say. Burning head could probably cause some mass panic as it should hit multiple units, and they dont have enought armor to make metal a good choice. The biggest threat in any skaven list is probably gonna be the abomination, so Pit of Shades and Beast Cowers are good choices. Mass poison from skinks should be able to handle it tho so I wouldn't pick a lore just to take out the abomb. 6 T5 wounds with regen is a pain, but Fire got some pretty good spells against it, a simple fiery blast should do around 2 wounds. Heaven could be decent to kill hiding mortar weapon teams with comet.
Tomb Kings - Fire. Some characters are flaming which makes Bane Head + Fire wall sooo much fun. The magic missles are also great against chariots and regular infantry. Rune of burning iron is also great with bane head (4 wounds in one go, one-shotting any Kings) but usually they dont have more than a 4+ save anyway and the other metal spells are pretty weak against tk.
Vampire Counts - Metal or Fire. Most VC players take a big unit of Black Knights with several characters in it and sometimes a regen banner. With SoTF you one-shot this pesky death-star unless he spends 6 turns hiding it. Vampire Lord sniping with Burning Rule and bane head is great aswell. If you dont face a BK bus (or blood knights) then Fire is a good choice, lots of good magic missles to kill wraiths, spirit hosts and infantry.
Warriors of Chaos - Lots of armor so Metal is awesome. The powerful WoC lists have level 4 generals, and with Chaos armor and mount they have 2+ save, Bane Head + Burning Iron = dead gate way spammer.
Wood Elves - Fire is what I usuall pick. Wood elves are glass cannons, with only a single unit with good save, but metal is still an ok choice, especially against treeman lists. IIRC treemen have horrible Initative so Shadow for Pit could be useful.
Summary:
Armies with bad LD and no/little ITP: Death or Fire
Armies with war machine and lots of armor: Metal
Armies with devestating monsters: Beasts or Shadows
Rest: Fire or Metal.
) also do not TARGET a unit, so there's not magic resistance for those spells either. Basically, if a spell requires a unit to be the target of its effects, then the TARGETED unit gets to use its magic resistance. Otherwise, they don't get to use it against that spell, even if they're still affected by it. 
