NLTK (Natural Language Toolkit) is used for such tasks as tokenization, lemmatization, stemming, parsing, POS tagging, etc. This library has tools for almost all NLP tasks.
Spacy is the main competitor of the NLTK. These two libraries can be used for the same tasks.
Scikit-learn provides a large library for machine learning. The tools for text preprocessing are also presented here.
Gensim is the package for topic and vector space modeling, document similarity.
The general mission of the Pattern library is to serve as the web mining module. So, it supports NLP only as a side task.
Polyglot is the yet another python package for NLP. It is not very popular but also can be used for a wide range of the NLP tasks.
