Best App/web part to add a company job board using SharePoint Online with Modern Pages?

If you just want to put content out there then News is good. We use a List so that we can have all the metadata for each posting and also export it to our public facing site. It also allows people to search or filter on things like Department, Office Location, etc. 

I'd create a separate Comm site for the purpose, that way you can create a section dedicated to job posts by filtering to that site. You can probably do the same now with the new metadata options on pages by creating a news article type and requiring that to be filled out, then you can do the same as well.

But basically we do it with comings and goings, basically when new people are hired, promoted or let go/quit they show up in this section. You could do something similar with Job postings. You'll need to provide a way to demote the news via Flow or adding Promoted state to the view to be able to be changed so it doesn't show on the page when filled.