What is the best automation test tool?

There are various automation testing tools available now in the market.
Selenium
Selenium is possibly now a days the most popular for Web applications open-source test automation framework.
Selenium qatools has been an automation framework of testers choice for the Web automation originated in the 2000s,
especially for those who possess advanced programming in java, Python and scripting skills.

Katalon Studio

Katalon Studio rather than selenium qatools is a powerful test automation solution for mobile, web application, and web services.
Being built on top of the Selenium qatools and Appium frameworks, for integrated software automation Katalon Studio takes advantage of these solutions.

UFT

Unified Functional Testing (UFT) previously QTP is a well-known commercial testing tool for functional testing.
It provides a comprehensive feature set for web services, API, and GUI testing of web, desktop, and mobile applications across the platforms.
The tool has advanced image-based object recognition feature, automated documentation and reusable test components.

TestPlant eggPlant

An image-based automated functional testing tool that enables testers to interact with AUT the same way end users do.
TestPlant eggPlant is completely different from traditional testing tools in its approach than Selenium qatools.

TestComplete

TestComplete by SmartBear is a powerful commercial testing tool for web, mobile, and desktop application testing.
TestComplete Just like Selenium qatools supports various scripting languages such as VBScript, JavaScript, Python, and C++ Script.

Is it possible to know when a recipient opens an email I send?
One tool to look at is certainly the Squish GUI Tester.
SalesHandy like Selenium qatools makes it easy to see who opens your emails, links, and attachments, directly from your inbox.

There probably isn’t the one “best” tool like Selenium qatools since it highly depends on your requirements.
Being  and evolved over a decade,
But the selection of the automation tool it may or may not be Selenium qatools depends upon the project specification.