Well, against undead and Deamons you usually want to take Light, though a quick note before I continue is that Slann usually excel best when you grab the 1st spell from different lores or a good roll you can keep, which allows for a nasty arsenal that will keep your enemy on his toes.
So, undead and Deamons, Light.
High Elves I'd suggest the Lore of Death and Shadow. (Btw, if you take a JSoD you may find him ridiculous with Steed of Shadows, 1st spell on Shadow)
Against Wood Elves, obviously Fire, ESPECIALLY if there are tree people about. Another would be if you're playing with hopefully a little more then that one wood the WE's bring, taking Life might not hurt, cause I bet you'd give a nasty surprise to you opponent if you used Master of the Wood and turned his terrain against him.
Tomb Kings you might take ONE Fire spell just in case for a lucky shot against a mummy. And always at least try a little to get Pit of Shades, it'll automatically sink those chariot units of his.
Against Dark elves I'd probably suggest Death, Fire, Metal, or Shadow.
Against Ogres, probably Fire since it can dish out lots of devastation, maybe Shadow if you think you can get Pit of Shades to Auto-kill the slow ones.
Hm...who else, ah yes, Beasts of Chaos. Well...once again really any Lore will work, though Beasts might help you quite a bit with The Beast Cowers spell making their numerous monster units in a fix.
Vampire Counts, Metal can work because the first spell is a single targeting flaming attack that increases in Str off of his Armor save, and if he dies, they lose. Though typically the VC's have some Regens up their sleeves so I pack Light, Metal, and Fire to smack them with.
A quick note here, if your opponent likes to hide in terrain or use alot of terrain pieces, Lore of Life is the way to go.
Against Empire, really anything except maybe Light because humans are rather fragile and if you mass easy-to-cast D6 Str4 spell he'll be in a fix.
Bretonnia, same as above, though probably Metal due to knights galore.
Another note, Heavens is beyond dangerous with a Slann if you get Uranon's Thunder Bolt and Forked Lightning, even the Comet, because usuing Telepathy with a Skink Priests armed with the cloak of Feathers means you have eyes everywhere and those spells have no range, so let Hell break loose.
Dwarves are rather easy to smack if you buff up enough. I'd suggest either Heavens and getting Comet + Uranon to beat they're range and armor saves, or Metal so that you can freeze their war machines and use those armor saves against them, just watch out for cannons!
Skaven, ew, I'd probably suggest the damn most destructive magic you can get your hands on. Fire would probably be wise, Death might also help. Once again, Life is amazing IF under the correct circumstances. Imagine his horror if you managed to use Master of Stone while he commands a hill!
Orcs and Gobos. I'd suggest Beasts and maybe something Ld attacking like Death with the Doom & Darkness spell, which would hurt those orcs. I'd say get Steed of Shadows and smash down his general with a 38' charging JSoD. Or maybe try for the Death lore and use spells like Wind of Death which could wipe out a decent unit with Panic. And once those Orcs start running...well...you know
And Lizardmen...well, I'm not really sure. Probably Fire though, which would give you some strong spells to blow up skinks quickly and others to wipe out Kroxigors. And if your foe chooses to send something big like a Steggy or Slann, simply do whatever it takes for Pit of Shades and watch him crumble in one swift blow.
Most important thing, KEEP YOUR SLANN SAFE!! I personally let my roam lone wolf style and hide him in some terrain or behind a hill with a Flying Priest to give me eyes. My JSoD wipes out fast-moving threats and my army makes sure to finish off and straglers. Just don't be dumb, and all will be well. (Keep in mind I use a fully-powered 2nd Generation toad, makes him a downright bitch to dispel, best of luck)
-Dalkarius
P.S. Beware of Fear causers!!!