Can't say I've seen enough to be confident on the best library but redux (https://github.com/reduxjs/redux) is just so simple, and has great, readable/understandable code.
const createStore = (reducer) => {
let state;
let listeners = [];
const getState = () => state;
const dispatch = (action) => {
state = reducer(state, action);
listeners.map(listener => listener());
};
const subscribe = (listener) => {
listeners.push(listener);
// unsubscribe
return () => {
listeners = listeners.filter(l => l !== listener)
};
};
// populate initial state
dispatch({});
return { getState, dispatch, subscribe };
};