The first sign that Moshe showed Paroh that Hashem was ruler of the universe was that he threw his stick to the ground and it turned into a snake. The Kotzker has a short vort where he says that this sign was to prove that Klal Yisroel was the chosen nation. He doesn't explain what about the stick turning into a snake proves that.
I was thinking we could understand this with a Rashi in last week's Parsha. Rashi says that the first time Hashem told Moshe about this sign it was a sign to Moshe that he spoke lashon horah about Klal Yisroel. The sign to Paroh was that Moshe who only spoke about Klal Yisroel was punished, so too Paroh who enslaved them would definitely be punished.