NLTK is widely considered the most comprehensive Python library for NLP. It is an essential library that supports tasks like classification, tagging, stemming, parsing, and semantic reasoning.

Other useful libraries include NumPy, Scipy, Scikit, PIL/Pillow, SimpleCV, Mahotas, SimpleITK, pgmagick, and Pycairo. Each library has its own unique set of features, advantages, and disadvantages.