Well we have some disciplines which are really awesome and some which are worthless/nigh worthless and some which are situational
Really Awesome
Focus of Mystery: Sometimes you don't need lore mastery, but most people when they pick a lore have a particular spell in mind they really want.
Focused Rumination: Extra power dice are always nice and are what pushes are magic phase over the edge. Especially useful if you are planning to cast lots of little spells in lieu of a few big ones.
Becalming: So good at nerfing enemy wizards, I agree with an earlier comment I saw that we are likely to lose it next edition. I play dwarves a lot so I haven't used this much myself though. Technically situational because you don't always have a wizard you need to neutralize (without killing)
Situational
Higher State of Consciousness: Good if you want a solo Slann. I think I like Divine Plaque of Protection for the protection against spells and magic artillery. I guess for the really paranoid you can take both.
Soul of Stone: The miscast table is so bad, I think the hard part would be deciding whether a reroll would be a good thing. I'd hate to avoid a Power Drain only to get a Dimesnional Cascade. I view this as a last line of defense, I'd rather use Cupped Hands, Throne of Vines, or simply conservative dice rolling to mitigate miscast issues.
Unfathomable Presence: Magic resistance was nerfed in 8th ed but it applies to the whole unit so you can protect the Slann's Temple Guard with this discipline if you want (though it provides nothing to the Slann)
Worthless
Transcendent Healing: Regeneration and Ward Saves don't stack anymore, this is only good if you are fighting enemies who ignore Ward saves for some reaosn.
Harrowing Scrutiny: Terror and Fear only works if you are in base to base contact in 8th edition so a Slann in a TG bunker can't benefit from this. A lone Slann won't gain much from causing Terror. You can have a TG bunker do this for cheaper by giving the unit champion the Terrifying Mask of EEE or the shrieking blade if you are strapped for points.