WordPress has a fairly strong mechanism to 
Although power users (technically sound) can use several captcha solutions that don't require a plugin and are highly customizable, an average user must use one of these plugins to ease the entire process.
Always test a captcha plugin on a demo page or a private page and make sure it's appearing correctly on mobile devices as well. Some captchas flow out of the container on smaller devices. So do check it.
1. Captcha Code - This plugin can be used to add captcha code protection on various types of logins and forms active on a typical WordPress site. The configuration of this powerful plugin is quite easy. Tt's translation-ready and can be easily used on multilingual websites without any issues.
As an administrator, you can decide the type of captcha as well as the placement of captcha within a website. It has a very simple interface and even a layman can configure it without any complex steps. I've used it on many clients' websites and they've reported almost nil spam after its activation.
2. Cartpauj Register Captcha - This handy plugin adds a captcha challenge to the user registration form of the site. It also adds honeypots in hidden format to deter various kinds of spambots. and last but not the least, it doesn't require any kind of complex setup to make it work. Just install it and activate it.
If you want a more rigorous spam protection system, you can use captcha code validation instead of using a honeypot. This plugin can also be used to change the logo on the login form. One can use it on older versions of WordPress too. This plugin is best suited for technically challenged.
3. Really Simple CAPTCHA - This plugin is used by a large number of bloggers to combat trolls. It was originally made to work with a much popular Contact Form 7 plugin. Users can easily integrate it with other plugins through simple PHP directives. This essentially means that you can easily extend it.
Its high user base suggests its popularity among its users. Developers can easily extend and customize it as per the site's needs. Instead of using WordPress sessions, this plugin uses temporary files to manage captcha instances. If you get a simple and low volume of spam, this plugin can stop it easily.
4. reCaptcha by BestWebSoft - This anti-spam plugin adds an effective captcha solution offered by Google to various types of forms on a WordPress blog. It also supports Contact Forms 7. It is multilingual with support for over a dozen popular languages and also comes with RTL support.
It supports both versions 2 and 3 of Google reCAPTCHA. It's compatible with all the popular form plugins as well as with the page builders. If required, one can manually specify the captcha language. Multisite installations are also supported. I often use this plugin on the login page of the client's site.
5. Advanced noCaptcha & invisible Captcha (v2 & v3) - This plugin supports both visible and invisible types of captcha. And, it also supports all popular form extensions as well as CMS extensions like BuddyPress and WooCommerce. Multiple captchas can be embedded on a single page.
You can also configure conditional login captcha through this plugin. So that the captcha appears after a fixed number of unsuccessful login attempts. The theme, size of the captcha, and the version can be selected at the time of configuration. The captcha works even if JavaScript is disabled.
6. Contact Form 7 Captcha - As the name suggests, this is made for integrating captcha systems in web forms created by Contact Form 7 plugin. So basically, it can be treated as an extension to the form plugin itself. It has a large user base with a high rating suggesting its popularity and reliability.
If required multiple captchas can be integrated on a single web page having multiple Contact 7 forms. Shortcode-based embedding process ensures even a technically challenged user can integrate it easily without any issues. The settings are dead simple to use which makes configuration a smooth process.
7. WP Captcha - This flexible plugin supports different types of captchas one can integrate as per the requirements. Apart from regular Google reCAPTCHA, one can also use mathematical, image, or icon captcha systems to the forms. You can embed it in different types of WordPress forms.
While using a mathematical captcha, you can specify the type of basic math operation a visitor has to complete to submit the form. Similarly, there are tons of customization options for both the image and icon captchas to fine-tune the level of difficulty of the same. It's one of my favorites.
8. hCaptcha for WordPress - It's one of the unique captcha plugins mentioned in this list. It rewards the web publishers while the visitors use this powerful and flexible captcha plugin. It uses the good old challenge-response system to distinguish between a human and a machine.
It supports a huge number of form plugins giving you easy options to integrate it with your favorite one. You can also integrate it seamlessly with popular page builders, eCommerce, and membership plugins. Its source code is also available on GitHub in case you want to contribute to its development.
9. WC Captcha - It's a mathematical captcha plugin you can use on different types of forms on your WordPress site. The captcha system automatically hides for logged-in users. Multiple captcha systems can be used on a single web page in separate web forms.
Apart from selecting the type of mathematical operation for the captcha, one can also configure if the captcha should display words or numbers. It can also block direct access attempts of core WordPress system files by the bots. One can also configure the input threshold time for the captcha.
10. WP Advanced Math Captcha - This is yet another powerful mathematical captcha plugin for WordPress users. Advanced users can also hide the captcha for a specific IP address range. You can also hide captcha based on the geolocation of the users which makes it quite flexible and user-friendly.
Like other competitors, this one too offers customization options for the type of captcha to display to the visitors. Website owners can see the statistics of blocked attempts in the form of a nicely formatted graph. The geo and IP filtering options give it an edge over other similar plugins.