Materials and Shaders for Avatars

Default Shader
The standard shader is tailored to work on realistic hard surfaces, not features that are found on many character models such as skin, hair, eyes or cloth. If you are going for a photorealistic style, it might be best to seek out more specialized PBR shaders to suit your needs.
Quest Shaders
VRChat’s shaders, like Standard Lite and Toon lit, are required for use on Quest Avatars. Outside of this use case they should generally be avoided since their feature set is vastly reduced for often minimal performance gain on PC (if not using lots of special effects on other shaders).
Toon Shaders
Two of the most popular and versatile toon shaders for avatars are Poiyomi's and lilToon.
Poiyomi has many features and has an extensive written and video documentation.
lilToon also has many features. It has documentation both in English and in Japanese.
Silent's is another option. It also has many features and a written documentation.
You can try them out for yourself and see which one you prefer.
Heightmaps
Heightmaps should generally be avoided on avatars. (They break at intense angles plus are more performance intensive. They are more ideal for certain use cases in environments, not typically on characters.) For more info on heightmaps, look at the Materials for Worlds page.
Thickness map
Used for subsurface scattering (SSS), to simulate when light passes through a material. A great example of when to use it is for skin, leaves or cloth.
You can generate this map by inverting the normals and baking ambient occlusion in Blender.
You will need a custom shader to apply it, like Poiyomi, or both Silent's photorealistic SSS and toon shaders.