I highly recommend Avatar 3.0 Emulator by Lyuma for any avatar creation as a must have. This will let you emulate the avatar within the editor in Play mode.
See: https://github.com/lyuma/Av3Emulator
Disclosure: I (Haï~) have participated in the development of this tool to implement the menu emulator.
I recommend Pumkin's Avatar Tools by Pumkin as a situational tool to copy components across avatars. Most of the time, I use this to copy Dynamic Bones, Constraints, and other components when I need to re-export a FBX after purchasing an avatar.
See: https://github.com/rurre/PumkinsAvatarTools
I highly recommend CyanEmu by CyanLaser for any world creation as a must have. This will let you emulate the world within the editor in Play mode.
See: https://github.com/CyanLaser/CyanEmu
I highly recommend VRWorldToolkit by 1 for any world creation as a must have. This will warn you and suggest fixes for many issues and mistakes during world creation.
See: https://github.com/oneVR/VRWorldToolkit
I recommend WorldCreatorAssistant by Varneon for any world creation as a nice to have, most likely when starting a new project (but it can be used on existing projects). This will download and setup a project for you.
See: https://github.com/Varneon/WorldCreatorAssistant
If you are used to SDK2 world creation, or if you need Udon for straightforward needs, you may choose to try CyanTrigger by CyanLaser. This provides an interface similar to SDK2 that lets you wire up simple logic such as synced toggles.