Of all the content management systems to use to build a website, why Wordpress?
Here's why...

Wordpress is built on open-sourced software making it possible for you to install for free, edit and customise however you like. Having this level of control and ownership over your website, including its design and files, gives peace of mind to those who know how to use it.
Because it is open-sourced, everyone has access to its code making it one of the fastest growing CMS's out there with a community of developers making suggestions and creating tools and extensions that can enhance Wordpress and its functionalities. 
With the way Wordpress is set up, installing it won't force you to rely on a third party like Wix or Squarespace but rather you have complete control over how you want to host, manage and design your website. 

Wordpress has been used succesfully for years, powering over 43% of all websites on the internet. With a track record like this and a huge community constantly improving it due to the open-sourced nature of the company, I think it is safe to say that Wordpress will remain one of the most reliable content management systems that you can use to build completely custom websites.

As I have mentioned, Wordpress can be used to design pretty much anything. With some knowledge of code and plugins, there are no limitations to what you can do with it. Having this level of customisation, also comes with its difficulties as it can be harder for beginners to learn how to navigate the backend but this can be simplified if you are working with a web designer as they can show you what you need to know and even modify the backend to make it as self-explanatory as possible.

With this amount of freedom in design, it is important to make sure that the website is still functional. Thankfully Wordpress is a powerful tool that can be used dynamically, edited and extended with code and plugins. With a big team behind it, Wordpress receives patches and updates regularly to prevent bugs and ensure that its code is running smoothly.

So now that you have heard me talk about the all of the possibilites to customise and design your website, you might be wondering what type of website Wordpress can be used for. Well, you own the website and 100% of its contents so its up to you. It is a flexible tool that is well suited for basic, static sites used for portfolios and brochure-style websites where you can share your work and biography with the aim of encouraging people to get in contact. It is also well suited for complex, larger websites that might be needed to platform an e-commerce or blogging website where there are hundereds of pages with dynamic content. This range of things it is capable of is unique in this sense of freedom to build anything with the only boundary being your knowledge and creativity. 

You can add/edit as many security features as you like. You have complete freedom over how you want to best secure your website from malicious activity and  hackers. With most website builders, you can't choose and customise security to the same degree. 
Wordpress itself pushes security patches regularly to make sure it is up to the newest standard. The same can be said for most reliable security plugins as they often update their code to prevent it becoming vulnerable to hackers. 

WordPress is written using high-quality code in semantic markup with built-in SEO capabilities. This means search engines will recognise your website easily and rank it well. 
You can also install various SEO plugins which can give you a more advanced insight into what you can do to improve your Google rankings.

Each CMS and website builder has its pros and cons. Some people might like the simplicity of a platform like Squarespace while others will feel restrained using it. It always depends on your specific use case and priorities.