Canada

The bulk of Canadian exports to the US are oil. We can certainly tariff US goods, but export tariffs on Canadian oil and hydro electricity to the US will probably be a big component of the Canadian response.

Even with a big increase on hydro power exports to the US, it's still cheaper than most other sources of electricity, so they'll still happily pay it.

As to oil, there are some refineries in the US that are set up to process Canadian bitumen. Canada's long history of just exporting raw materials and re-importing finished goods give a pretty outsized room to manuever here. This refineries will probably just pony up and pay more.

Where the damage will come is in areas like the auto sector, where there are tightly coupled supply chains.

Paul Krugman points out in his article The End of North America that there is probably already damage done to the concept of North American manufacturing.

Trump is going to do a whole lot of damage, we're going to take some of that damage, but my favourite quote on the topic:

no one wins trade wars, but the kid who never stands up for himself always loses and for a long time

Scotiabank Daily Points

Canada and Mexico account for about a third of US exports. US businesses may be able to survive without them, but they sure won't make the shareholders happy.