As a foreword, I try to make sure mods I pick both fit into the lore (doesn't need to strictly adhere to it as long as it looks like it belongs) and be balanced (to a certain extent).
Crynsos' Faction Unlocker+ - This mod is actually a framework that splits various lords (so Ungrim to Karak Kadrin, for instance) to various factions, adds some interesting stuff and allows for certain mods that would be otherwise mutually-exclusive due to startpos to actually work together. The only thing I dislike about it is that it makes Isabella into a hero. Despite the name, to actually unlock the factions, you need a host of submods.
This one is for the vanilla factions, and in its description links to the rest can be found. Don't use too many at once because it can clutter the interface.
Cataph's The Southern Realms - Pretty much a classic at this point. The unlockers above let you play as them. Anything made by Cataph is pretty much guaranteed to be good.
Cataph's Southern Realms Regiments of Renown - Requires both the Southern Realms mod AND the Faction Unlocker. Adds the Regiments of Renown from the old Dogs of War tabletop armybook.
Cataph's Wizard Lord Campaign Buffs - Makes Gelt and Kemmler stronger.
Cataph's Bretonnian Character Skills Tweaks - Your lords start being able to recruit regular knights, horses (except for damsels and the enchantress) unlock on level 2, lance formation is forcewide instead of just the character, Alberic is better, Fay Enchantress has two lores like Manny.
Cataph's Closer to Tabletop - It should be clear at this point I'll grab pretty much every mod by Cataph.
Cataph's Bretonnian Foot KnightsCataph's Kraka Drak: The Norse Dwarfs - New faction with a bunch of goodness, and a really hard start.
Cataph's Magic: Closer to Tabletop - Magic is good.
Cataph's Lance Formation - Just gives it to Bretonnia units that didn't have it.
Cataph's Greenskins Teef Currency - Just a little bit of fluff.
Cataph's Chaos Steeds Resized - No Chaos ponies.
Ultimate Chaos - I took this for a spin and frankly like it better than Chaos Divided. The point is pretty much the same, but it allows and encourages themed armies.
Sleepy's Southern Hordes - Updated - More units for Greenskins. Not much to say.
Mixu's Mousillon Unit Pack - Crynsos Edition - Adds some units, and Krell!, to the ol' city of the damned.
Empire of Man - Crynsos edition by Mixu - Fleshes out the various counties of the Empire with unique units, and especially Middenland. Keep in mind that a special barracks is added to each province that allows using the units from it.
Empire of Man - Additional units mod by Mixu - A few more units, some of them RoRs for the empire factions.
Legendary Boris Todbringer - Overhaul's Comrade Boris' skill trees a bit, mostly adding items.
Heroes of the Empire - Adds three heroes: the Amethyst Wizard, the Gold Wizard, and the Imperial Engineer. And it is awesome.
Norsca Overhaul - Crynsos Edition by Mixu - Fleshes out Norsca a bit.
Raze and Revel - New Settlement Occupation Options for Chaos - Makes Chaos fun to play, or at least more fun. It adds beastmen-like conquest options. Raze and get loot, or raze and get replenishment.
Better Black Orcs - Just makes black orcs look black with various white patterns on them.
Better Night Goblins - Another visual change to make them more diverse.
The Tomb Kings - Custom battle pack - Adds the Tomb Kings to custom battles. Really well made. No idea why the author chose the Hierotitan to be the sole lord.
Numas - A pretty battle map to go along with the above. Has actual pyramids.
Grand Campaign Custom Maps - A compilation of custom maps to appear in the campaign. It has links to the various required sub-packs.
Starting Traits Remastered - Lords are no longer blind, lame and so forth on recruitment, instead having more appropriate traits (some of them negative still).
Trident for Alberic with vanilla helmet - Just gives him his quest weapon. There is a version without the vanilla hat, but I like it.
Prometheus Skill Points Renewed - Lords get double points starting from level 10, heroes get double points every 5 levels.
Gargoyle - Adds a HeroQuest monster to the Warriors of Chaos.
Skeleton Warriors (Scythes) - Another HeroQuest unit.
Guv's Knights of Sigmar's Blood - Unit for the Empire. Sigmar Vult.
Guv's Chaos Dwarf Warriors unit - What it says on the tin, for Chaos.
Guv's Chaos Dwarf units Add-on - Adds two more Chaos Stunties to the WoC.
Guv's Chaos Feral Manticore/Harpy lord skill fix - Turns out the Freakish Mutation lord skill doesn't affect these two. Now it does.
Guv's Free Company Archers unit - Requires the Grim and the Grave, take regular free company, trades armour piercing for range and firing arcs.
Guv's Gor Great Weapons Herd unit - What it sounds like.
Guv's Dual Wielding Black Orcs unit - Unfortunately not affected by the black orc visual mod.
Guv's Shielded Big 'UnsGuv's Grave Guard Halberdiers unitGuv's Black Knight Hellsteeds unitGuv's Shielded Halberdiers unitGuv's Dual wield Bestigor Herd unitBetter Trade - Changes how trade cap works. In vanilla, it is a flat 12. With this, for every region up to 5, you get 4 more exports allowed. After 5 you get diminishing returns up to 50 exports at 20 regions.
Mousillon City of the Damned: No Lords - While there are several versions, this is the only one that doesn't touch the other VC factions. Makes Mousillon very pretty.
Awaken fro mthe Grave spell for Kemmler - Show Ghorst who is the real necromancer here.
No Aggressive AI Agents! - Even if you ignore everything else, grab this.
Coven Throne - Coven Throne for VCs. Needs G&G.
Resized Lords and Heroes - No more giant Karl Franz.
All Tabletop Lords - I really, really like this mod. Only factions without lords from this are Chaos and the WElves.
All Tabletop Lords - Resized Lords - As with the normal resized lords.
Regiments of Renown! - Adds more RoRs to Dwarfs, Greenskins and VCs.
AI colonises more often - What the name says. Be careful, though, since the AI will rush to colonise if they are close to a ruin... even if it used to be your city a turn ago!
Dem Orkz! - Makes orcs more colourful.
Skip Intro LogosBetter Combat Camera - Up Close and Personal! - It also lets you zoom out more.
More Board VariationVaried GeneralsBuilding Progression IconsDiplomatic Affinity Overhaul -Changes AI personalities around, making them more interesting.
Tier 4 Minor Settlements - Extremely nice, and the AI can use it too.
Heinrich Kemmler - Corpse Cart of Omen Mount - More Kemmler buffs! More!
Dark WaterBetter Campaign Map - This and the above make the map even more good looking.
Adds heroes to level 2 garrison buildings.Kiselv Withstands - Unfortunately just a reskin.
Realistic Climate Battle Change - It's pretty.
Improved Loading Screen Artwork - Pretty, I say!
Better Auto ResolveHome Region Movement Bonus - This is really nice, gives armies more movement in their owned territory. Helps chase down norscan stacks.
Storm of Chaos Mini Campaign - Yes, you read that right. Keep in mind that you need Call of the Beastmen to run it (it replaces that campaign) and, very importantly, while it does work with the rest of this list you really should run it on its own.