If I am a dual citizen of Canada and the USA, which passport do I?
If you have a passport from the same country as the agent asking you for a passport, always show that one. It is always better to be on the same team. In the case of the US at least, this is mandatory; you must use a US passport to enter if you have one. If you do not have a passport from the same country (you are entering a country besides the US or Canada), you can show either, and ideally the one that will get you better treatment. When leaving a country, you should always show the same passport you entered with. There is no good reason to ever reveal, to any immigration agent of any country, that you have more than one passport.