Hi, here are my developing WordPress plugins which I use in my daily routine:

To see the queries and loading scripts/styles in my WordPress site, I use:

Query Monitor: https://wordpress.org/plugins/query-monitor/

Plugin to see whether everything is fine with our WP theme. It runs a quick scan (the same as doing a spell check before submitting an article):

Theme Check:  https://wordpress.org/plugins/theme-check/

Password protect your whole WordPress site with a 1 password (you can use it as Under Construction WP plugin alternative). You can also allow (whitelist) your IP address 🙂

Password Protected:  https://wordpress.org/plugins/password-protected/

To import dummy WordPress pages/posts/categories/comments/tags, etc. (Test Unit Data) I use: https://codex.wordpress.org/Theme_Unit_Test

Or you can add dummy content from http://wptest.io/

For my WordPress debugging I am using the official plugin from WordPress.org: Debug Bar: https://wordpress.org/plugins/debug-bar/ ...and his add-on for actions and filters: Debug Bar Actions and Filters Addon: https://wordpress.org/plugins/debug-bar-actions-and-filters-addon/