Find correct forms of a word I'm trying to find all grammatical forms for words from a dataset. I'm new to NLP.

I'm using a combo: spaCy (to get a base form of a word), word_forms (to get all possible forms). Sadly, it seems that they are not able to get everything right. Examples like 'wildest' or 'scariest' seem to be out of reach.

Maybe I'm expecting too much (in most of the cases the solution works really well).

Is there a decent tool I could use? I'm aware of nltk lemmatizer, its performance is worse than word_forms and spaCy. I'd appreciate some kind of advice.
Out of curiosity, have you made any effort to quantify the accuracy of spaCy?  The way that it performs lemmatization leverages the syntactic similarity hypothesis, and as such 100% accuracy is an unrealistic expectation.

With that said, if there is a particular subset of words that are essential for you correctly classify, and you're willing to create a labeled training set, spaCy allows you to train custom models. I believe that you are able to train these models based on the existing language models, and so the amount of training data need not be prolific.