I am giving you the honest review of all the three plugin so that you can decide with yourself that which SEO plugin best suits for YOU. I have not taken any money for any of the SEO Plugin thats why for me all the plugin is same.

Best Free WordPress SEO Plugin

Type of Website	Recommended SEO PLugin
Small Website/Beginners Website	Rank Math SEO Plugin
Medium Website/Professional Website	All in One SEO Plugin
Enterprise Website/Large Website	Yoast SEO Plugin

Unique Features Rank Math SEO Plugin

As according to my Opinion For Beginners and For Free Rank Math Plugin will be best because of some benefits in free version of Rank Math SEO:
a. It has the rank score which you have to score 100 out of 100 and in this you will learn a lot of SEO of Your Content. And You got 100/100 means your Article has Good SEO.
b. It provides a Unique Features of Instant Indexing using Google Cloud API.

Unique Features All In One SEO Plugin

Ans As According to my Opinion For Medium Website All in One SEO Plugin will be best because of some of the benefits of Free Version of All in One SEO:

a. Given the Option to Write the Separate Title,Description and Keyword Option of all of the Articles. 

b. Option to create both sitemap.xml and sitemap.rss.

c. You have the Option to Set Priority and Frequency of your Article.

Unique Features Yoast SEO Plugin

Ans As According to my Opinion For Enterprise Website All in One SEO Plugin will be best because of some of the benefits of Free Version of All in One SEO:

a. Option to Write focus Keywords for Your Article.

b. Step by step guide to improve the SEO of Your website.

c. Sitemap for news is separately available.

d. Show Various factors to Improve the Readability of Articles.