Our northern warning system needs to be completely renewed. That's a big expenditure. Canada should take the lead. That's directed against air-breathing threats, including cruise missiles from really dangerous countries like Russia.
Rob Huebert very correctly mentioned the Canadian surface combatants. If you want to have a discussion about missile defence, talk about whether we should put Aegis class radar and missile systems on those Canadian surface combatants. Now is the time. It will cost this country $10 billion to do that.
If you had unlimited defence budgets, you could have perfect defence. You don't have an unlimited defence budget, so you have to prioritize.