You have lots of options in terms of magic:
Slann: any of the 8 main lores from the main rulebook, lore of high magic or all 8 battle magic signature spells.
Skinks: Lore of Heavens or Beasts
In terms of the Slann, the most popular of the BRB lores are:
death, life, light, shadow. Additionally, taking the 8 signature spells (via Wandering deliberations) or loremaster
High magic (via focus of mystery) are very very popular.
If you want to take magic that is unique to Lizardmen, then your only option is to take High magic. Lizardmen are among only three armies that can access it (High and Wood elves being the others) and we have a unique and very useful lore attribute.
My personal favorite for the Slann is taking the Lore of High magic with Focus of Mystery (loremaster). Having all 8 spells and the ability to swap out spells via the lore attribute is extremely versatile. It does require a bit more planning/strategy, but can be used to devastating effect. It contains some really useful spells and can address just about any phase of the game. You can watch a great tactica video on using High Magic here:
You can find some great information about the different spells of the various lores we have access to here:
http://www.lustria-online.com/threads/the-8e-lizardmen-handbook-updated-november-2nd-2014.13130/
Hope that helps