I want to create animation videos very similar to the following videos:
From your experience, What are the best tools to learn to do that?
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I would strongly suggest taking a look at krita which has the following points in its favor:
Krita is a free and open source cross-platform application that offers an end-to-end solution for creating digital art files from scratch. Krita is optimized for frequent, prolonged and focused use. Explicitly supported fields of painting are illustrations, concept art, matte painting, textures, comics and animations. Developed together with users, Krita is an application that supports their actual needs and workflow. Krita supports open standards and interoperates with other applications.