Unity 2019.4.39LTS Build-In render pipeline

HQ FPS Weapons
This has got some mixed reviews on the asset store. Mostly people feel that the dev's moved on to his newer Survival template, but I found this easy and fast to get up and running with. There's almost zero documentation so you'll need to know your way around somebody else's code if you want to change it in any way, but it's all very well put together under the hood. There's also a good community discord if you get stuck. The art assets are top notch too. I'd say there's a lot here for your money.
https://assetstore.unity.com/packages/templates/systems/hq-fps-weapons-177306?ai...

Aura 2 - Volumetric Lighting & Fog
Now showing its age, not just because it's Built-In only but also due to some of the 'streaking' visual artefacts, but still a classic. 
https://assetstore.unity.com/packages/tools/particles-effects/aura-2-volumetric-...

NGSS2 (Next-Gen Soft-Shadows)
This so needs an update to URP and HDRP. I hardly ever use it (this was essentially a resurrected legacy project) because it involves overwriting Unity's core shadow libraries, but in Built-In it really shine. Well worth joining the discord too. It has some super talented technical artists on there.
https://assetstore.unity.com/packages/vfx/shaders/next-gen-soft-shadows-137380?a...

Tropical Forest Pack
I so wish Boldinoboy would make more asset packs. This is truly stellar and sells at a giveaway price.
https://assetstore.unity.com/packages/3d/environments/tropical-forest-pack-49391...

Wasteland Cabin (free)
From Lukas Bobor, a talented artist who gives away all of his assets for nothing... Many free assets are average at best but this is beautifully modelled and textured. I would quite happily have paid for this quality
https://assetstore.unity.com/packages/3d/environments/urban/wasteland-cabin-9486...

RPG/FPS Game Assets for PC/Mobile (Industrial Set)
I used the v6 demo, but from the v7 set. (Each update includes all of the previous content. Also worth nothing that each update is a reasonable €4.46 and that v2 is free) You get a really good selection of industrial models for the price.
https://assetstore.unity.com/packages/3d/environments/industrial/rpg-fps-game-as...

Post Apocalyptic World Pack
Now slightly showing its age so not sure if I'd now pay full price for it but it's still a great example of a good game ready environment pack. Great for performance, and looks good in built-in. Some of the textures could do with upscaling and some models are a little too low poly, but that's ok.
https://assetstore.unity.com/packages/3d/environments/urban/post-apocalyptic-wor...

Desert Environment - Town & Palace | CITADEL
Quite mixed reviews on the store but in Built-In, it does ok. Some missing colliders, not the best textures, but cheap and cheerful. I think I picked it up in a sale or a bundle.
https://assetstore.unity.com/packages/3d/environments/fantasy/desert-environment...

Desert Buildings (free)
More Lukas Bobor freebies. Not as detailed as the cabin but decent.
https://assetstore.unity.com/packages/3d/environments/urban/desert-buildings-714...

Desert Buildings Modular (free)
https://assetstore.unity.com/packages/3d/environments/urban/desert-buildings-mod...

Desert Fortification (free)
https://assetstore.unity.com/packages/3d/environments/urban/desert-fortification...

Realistic grenade Pack (free)
https://assetstore.unity.com/packages/3d/props/realistic-grenades-pack-53211?aid...

3D Fire & Explosions
https://assetstore.unity.com/packages/vfx/particles/fire-explosions/3d-fire-and-...

Gun & Explosion Sounds
https://assetstore.unity.com/packages/audio/sound-fx/weapons/gun-explosion-sound...

indicator PRO
From echoLogin, a game dev veteran who actually makes games, and it shows in his assets. Easy to use, highly performant.
https://assetstore.unity.com/packages/tools/gui/indicator-pro-141266?aid=1100lMe...

CTS 2019 - Complete Terrain Shader
(Deprecated) Wouldn't go near this anymore. I'd recommend Jason Booth's MicroSplat instead.

Tactical AI
(Deprecated) Such a shame that this is no longer on sale. It's by far the easiest way to get a bunch of shooting enemy AIs. Obviously not as flexible as something like EmeraldAI but it does what it does really well.

Odin Inspector
Indispensable. It's one of those assets you don't realise you need until a little later on in your game dev career, once you start looking at tooling and custom inspectors.
https://assetstore.unity.com/packages/tools/utilities/odin-inspector-and-seriali...

DoTween Pro (there's also a free version)
Another must have.
https://assetstore.unity.com/packages/tools/visual-scripting/dotween-pro-32416?a...

Kronnect Shader Control
If you import a lot of different assets into a project, as I did with this one, you often run into issues with having too many shaders. This has saved my skin on more than one occasion.
https://assetstore.unity.com/packages/vfx/shaders/shader-control-74817?aid=1100l...

Google Resonance Audio
(Deprecated) These days I tend to use the Oculus Spatial Audio SDK but I didn't get a chance to update it

Mesh Materializer
(Deprecated) I really only use this for the Terrain To Mesh feature and the ability to terrain vegetation to gameobjects

The asset links above contain affiliate links, which means I may receive a commission if you click on and purchase something that I've recommended. It won't cost you any more money but it'll help me fund my own development projects while recommending assets I personally use and endorse.