I’m unaware of a free WordPress form builder, that features more functionality for free, than caldera forms. It includes advanced fields file upload fields, conditionals and calculations. The responsive form builder offers completely customizable row layouts.
Caldera Forms includes a Gutenberg block for selecting forms, and even displays the form content in the editor. The styling may not look exactly as on the frontend – as with most Gutenberg content – but it gives you a good idea whether you picked the right form.
Extract from WordPress.org
Caldera Form is a free and powerful WordPress plugin that creates responsive forms with a simple drag and drop editor. It has many free user-friendly add-ons for both beginners and web developers. The visual editor simplifies form building for WordPress. Anti-spam, AJAX, notification emails, and database entry tracking are enabled by default.https://wordpress.org/plugins/caldera-forms/
At the time of writing, this plugin did not pass the HTML validation test by the W3C
On this WordPress 5.5 test bench, activation of Caldera Forms throws this error:
Notice: register_rest_route was called incorrectly. The REST API route definition for cf-api/v2/entries is missing the required permission_callback argument. For REST API routes that are intended to be public, use __return_true as the permission callback. Please see Debugging in WordPress for more information. (This message was added in version 5.5.0.) in /home/oldrupdk/oldrup.net/wp-includes/functions.php on line 5225
Update: on WordPress 5.6 beta 3, Caldera still shows the previously mentioned errors, but it also has dificulties showing entries in the backend.