what is the best json form builder?

If by “JSON form builder” you mean a tool where you can visually build a form and get a JSON definition that your app can render, the strongest options are:

Tool	Best fit	JSON/schema	Visual builder	Backend
SurveyJS	Most complete embedded builder	Excellent	✅ Excellent	Bring your own
Form.io	Forms + APIs + workflows	Excellent	✅ Excellent	Built-in platform
JSON Forms	JSON Schema-first apps	⭐ Excellent	Limited	Bring your own
RJSF	Simple React JSON Schema rendering	⭐ Excellent	❌	Bring your own

My practical recommendation

For a new application, I'd start with SurveyJS if the requirement is specifically “let users/developers build forms visually, store the result as JSON, then render those forms in my app.”

SurveyJS's builder generates JSON definitions, supports React/Angular/Vue/plain JS, and lets you keep your own backend and data storage. SurveyJS+1

SurveyJS

Choose Form.io instead if the JSON form is supposed to be more than UI configuration—e.g. you want the form definition to also drive REST APIs, submissions, permissions, workflows, and self-hosted infrastructure. Form.io+1

Form.io

Choose JSON Forms if you already use standard JSON Schema as your application's data contract and want a clean separation between the data schema and UI schema. It supports React, Angular, and Vue. SurveyJS

JSON Forms

If you tell me your stack

For example React + TypeScript + JSON Schema + drag-and-drop builder + save forms to PostgreSQL, I can narrow this down to the best 1–2 choices and show you the architecture.