Q:

What is the best SE3 library for python?

I am currently googling on this topic but I would like to hear your opinion.
What is the best self-contained SE3 library that offers interconversion between quaternion, rotation vector, transformation matrix?
Those are the functions that I often use for my projects and all implemented in Matlab. And I am looking for an alternative to avoid re-implementing them all in python.
Quaternion2RotVector and its inverse
Quaternion2RotMatrix and its inverse
RotMatrix2RotVector and its inverse
SE3 inverse, the inverse function of each rotation representation 

A:

ROS's tf.transformations.py has self-contained code for doing these functions and can be used without installing ros.  In fact, the python code only depends on numpy!
transformations.py 

A:

I wrote my own and learned useful things about numpy and python object initialization in the process. I don't regret it.

A:

Writing your own package is always the best way to learn.  If you want to try something premade here are a few packages to choose from:

Robopy, Python3+numpy+scipy, also available from PyPy, this is an evolving Python port of my Robotics Toolbox for MATLAB
Sophus C++/Eigen with Python wrappers

