Which is the best extension to speed up Magento 2 sites?

I am getting some troubles with speed of my site because it loads very slowly. I have checked speed on GTMetrix and the result is not good. And I am also looking for the best extension to help me to improve this situation. However, there are a lot of extensions on the market place and I feel very difficult to know which one is the best? Can anyone suggest me an extension you are using and find it efficient to speed up Magento site?

I am using Magento 2

There is a very useful way to optimize Magento 2 site speed is deferring JavaScript loading till the last moment after all HTML and CSS. It means that JavaScript files are put to the end of page, therefore the web browser will load HTM and CSS before loading these JavaScript files. This leads to loading time be reduced and magento site perform faster than before.
To make this JavaScript parsing easier, we had better not do it manually, let's use an extension instead because of convenience and simplicity.

With Magento 1, Defer Javascript extension is also very useful to speed up websites