Each of the sentences is natural, but there are a couple of things you could do to improve it as a speech. 1) Each of the sentence is really short. It would sound a little better if you had at least one longer sentence, for variety. 2) When you say “return the favor,” will the audience know what favor you are talking about? If not, you should explain. 3). When you say “keep doing you best,” will the audience know what you will keep doing your best at? If not, you should explain.