For the React environment, I would recommend React Spring, Framer Motion, React Motion and React Transition Group.
If you work with Vanilla Javascript and CSS, some of the first ones that comes to mind would be AnimeJS, AnimateCSS, HoverCSS, MoJS, PopmotionJS, DynamicsJS, ProtonJS, KuteJS, ParticlesJS and ThreeJS.
What are your favorite animation libraries?