I feel that you have a lot of options in lores with a Slann, there is really not simply a best choice. I feel that there are 4 main choices to build your Slann lore wise, but it is possible to deviate even from these choices and still have a ''meta'' list.
- High magic gives you the most options, but it is also the most expensive as you should buy the loremaster upgrade to really benefit from this lore. Core in this lore are Walk Between Worlds, movement advantages is the thing warhammer fantasy is won on, and the ability to swap out spells. This will require a lot of experience, i would say this is the hardest lore to use optimally. Simply picking on the fly what lore you might need is hard, but rewarding and strong.
- WD gives you also a lot of options, but not as many as High Magic and the spells tend to be slightly lower on impact. You won't very likely win a game on a single spell, but you will be casting a lot of spells. Core spells are Wyssans Wildform to give your units very strong combat buffs and Searing Doom to kill high priority targets in the current meta. It also has the added benefit of having very low casting values, so this might be the best lore in a TG bunker due to the low chances of miscast. The biggest enemy of the TG is still the Slann.
- The Lore of Life is a lore that goes especially well with our monster due to the healing passive and the single target buff spells. If you want to save some points on your Slann and if you have a lot of monsters, this would be my go to. It also has one of the best nuke spells in the game in the form of Dwellers from Below, do not shrink to combo Throne plus 6 dice Dwellers if you get the chance. I feel that Earth Blood and Dwellers from Below are the core spells in this lore, both have massive changes in every game. I feel that Throne of Vines is overrated for non-loremaster mages as you miss out on a lot of spells if you take it, but i would still use it if you get the Throne/Dwellers combo on your roll.
- The Lore of Light buffs the general weaknesses of our strong core infantry. Sauri are great for their points, 2 attacks, S4 and T4, they just really get let down on their I of 1 and their WS of 3. That is just the thing the Speed of Light buffs, and this spell makes our Sauri into real CC monsters, maybe even more then Wyssans Wildform does against certain targets. Other spells also buff our Sauri, the Timewarp is a good example of this. This is my go to if i want to safe on my Slann while i have a lot of Sauri in the list. It has the added bonus to be very damaging to Undead and Daemons, especially the signature spell is very damaging to these armies. I see this as a bonus to buffed Sauri, but if you know you will be figthing a lot of Daemons and Undead (which is quite likely in this tournament meta) feel free to give this lore a bump up.
Other possible lores are the Lore of Shadow for massive nukes on low I armies while being pretty poor against high I armies. The Lore of Death is also not a bad lore as it has some good nukes and it is good at sni[ing characters, but if your opponent has high I or no high priority targets this Lore sees not as much usefull play. The other lores have some use, but are generally really outshone by other lores in strenght. I hope i gave you an idea about what lores are good on Slann, the message is that there are a lot of viable lores and the best lore for you depends on quite a few factors, including your army, your experience and your opponent.