Its really tedious but it will be all of them. But if you don't have the resources, just go over which SHOULD be more popular. Another could be selecting the ones that are already ranking on the bottom of the SERP and just needs a little boost to get to the top.
It could be tedious because just because your SEO a page, it doesn't guarantee 1st page rankings. Lets say 7th page to 4th page will not do much of a difference. But if it is to 2-3rd page to 1st page, you will definitely increase the traffic.
Its all a matter of whether or not the page will actually rank. Otherwise, it will be pointless.
So again, my suggestion is to optimize the better ranking pages that aren't at the top yet.