The first book in a series is often an excellent choice for an ebook promotion.
Introducing new readers to Book 1 gives them a natural starting point and creates an opportunity for interested readers to continue into the rest of the series.
Free promotions can be particularly useful for this strategy because they reduce the barrier to trying an unfamiliar author.
That does not mean later books can never be promoted. The best choice depends on your series structure, pricing, and goals. But if your primary objective is attracting new readers to a series, Book 1 is usually the most logical place to begin.