Best tool for text pre-processing, involving tokenization, lemmatization, stop-word removal, feature vector extraction?
Comparing two libraries or tools in terms of these things is somewhat that is opinion dependent. Some people prefer NLTK for doing almost all the tasks. spacy has also gained quite reputation. But what is better for you depends on what you want do. In my personal experience, I have found that NLTK along with gensim libraries is all that I need to do all the Natural Language Processing tasks.