What is the easiest way to remove all packages installed by pip?

Use the below command based on your OS:

For Linux Machine:

pip freeze | xargs pip uninstall -y

For Windows Machine or Linux Machine:

First, save all installed packages in a requirement.txt file, using the below command

pip freeze > requirements.txt

Then use the below command to uninstall all packages 

pip uninstall -r requirements.txt -y

If the above command may get failed if PIP is not able to uninstall any specific package due to some dependency and may throw error as below

ERROR: Cannot uninstall 'llvmlite'. It is a distutils installed project and thus we cannot accurately determine which files belong to it which would lead to only a partial uninstall.

Then just edit the requirements.txt file and comment out that specific package and save the file and execute the above again. [Workaround]

To deal above problem we can use the below script to ease the process.

For Windows:

foreach($line in Get-Content requirements.txt) {
    if(!($line.StartsWith('#'))){
        pip uninstall $line -y
    }
}

For Linux:

cat requirements.txt | xargs -n 1 pip uninstall -y