When this question first occurred to me, I assumed (following a mid-70s National Lampoon article about Canada) that the answer would be "tapioca". 8-)
And let's face it: the question assumes there IS such a distinctive thing as "Canadian cuisine" [what, poutine? what else?]
But after looking at the list of the most common ingredient in many cuisines, [via King],
Ms Eclectic and I agree that likely the answer is "butter".
But actually I think it's probably "water".