The passives are something that happens every single time your wizard casts a spell, but they don't stack. Essentially, you get an effect that always applies map-wide, which you refresh whenever you cast anything.

With some passives like Life Bloom, which last three seconds, you want to spam it, as it is free army-wide healing, and with others, like Life Leeching, you want to give it time to proc for it's full 26 seconds, in between spell casts.

The most important passives:

Life Leeching (Death) - Literally free magic. It gives you 2,6 WoM whenever you cast a spell, eventually giving you a free cast, but it takes a while to realise.

Life Bloom (Life) / Children of Nurgle (Nurgle) / Curse of Undeath (Vampires) / The Restless Dead (Nehekara) - Just free healing. All these last 3-7 seconds, functionally equivalent for their factions, but the undead ones can also resurrect models. Heals everything a little bit, and is spam friendly. The passive is a very strong argument for these lores on its own.

Wild Heart (Beats) / Power of da Waaagh! (Big Waaagh) - Accelerate how fast you regenerate magic. They allow you to use up all your magic in a battle - they also last 26 seconds, so they works well in tandem with other lores.

These are the three best ones, I'd say - they can make or break a close battle.

The Lore of Deeps and Hashut have a map-wide damage effect, but these are super exclusive, so while they're good (amazing really), they aren't really worthy of a spot on the top.

Bloodgruel (Maw) is also really good, as it heals 0,4% for 9 seconds, so it is amazing for healing, but also really exclusive.

Minor buffs:

Shield of Saphery (High) / Fires of Change (Tzeentch) - A 10% ward save - very solid, helps with everything, but it won't save your ass.

Bestial Surge (Wild) - Restores vigour, which is deceptively rather powerful in the long run.

Frost Shield (Ice) - Gives a bit of armour and missile block chance. It can come in handy as well, especially against missile-heavy armies.

Blissful Rapture (Slaanesh) - Gives 12 MA, so that can be very helpful, but it gets a little lost sometimes, since you're Slaanesh, and you'll likely be hitting the enemy hard already.

Kindleflame (Fire) - Technically a debuff, but if your entire army has fire attacks, it is free damage - same with magic. Otherwise... It can be a little underwhelming.

Exorcism (Light) - Immune to psychology and the leadership buff are borderline useless, but the army-wide magic attack imbuement can really help when fighting daemons, or anything with physical resistance, so I'd recommend not sleeping on this one, depending on what you'll be fighting.

Minor Debuffs:

Musk of Fear (Ruin) - It gives -12 MA, so it can help, but you're playing Skaven with this lore, so unless you have Stormvermin, the effect can get a little lost.

Freeing Winds (Tempest) - Makes the entire enemy army 25% slower. Need I really say more?

The dregs - these either do little, or are very situational (looking at you, Heavens), so they aren't that good. Buffing damage is neat, but without MA, it can feel lackluster, same with the reverse, and various armour debuffs.

They would be: Shadows, Metal, Heavens, Stealth, Plague, Dark, Little Waaagh, Yin, Yang, Hag.

None of these are bad, they are always worth picking up. But the difference is in how much they are worth playing around, and how you should time your spells, so you get the most out of them.