WordPress is the most common CMS platform for blogging and simple business websites. You can also customize WordPress to make it a portal for like travel agency or may be for job portal. If you starting the WordPress website, then you must install these plugins. Below are the list of few free must have plugins.
WordPress SEO/All In One SEO Pack
Search engine optimization is mandatory and specially On-Page SEO is now much important by Google if you are making the new website. To help search engine to crawl and rank your website. You must install one of these plugin. Both of these plugins basic features are enough for On-Page SEO. They have option to add description, title, sitemap, option to add webmaster tool verification code and analytics on your website plus some important other options.
WordPress has changed their editor for newest version. But I believe the classic editor is the easiest to use and this editor plugin is still downloaded by the hundred thousand users. You must install classic editor plugin in your website.
Your website probably get lot of comments if it start to get the users. Some time spammer may start to add comments with their auto tool and flood your website pages with Spam links. Installing this plugin will secure your comment section to much extant
No Category Base
You might have already already thought the structure of your website. If you want to remove category from every category page URL then this plugin will work, even with the Multi-WordPress.
Contact Form Plugin
Contact form plugin is necessary if you want to have a contact page. You can have a nice little form and any one can sent message or contact through it. This will make your users to easily contact and you will get every message in your mail box.
WordPress Backup Plugin
You must download any WordPress backup plugin. There are many and you can choose it according to your need. They features like they will take backup weekly and upload auto to your cloud like google drive. It will help much if you loose your data any time.
Other Free Tips
I would recommend you not to install lot of plugin if you are using it for blog or may be eCommerce. As it could slow down your website. Try to use WordPress in any SSD web hosting and use CloudFlare to make it more secure and fast.