Reporting, analytics, email marketing (workflows), forms, blogging. These would be my top pics and here's why:
Reporting is very straightforward, detailed and yet easy to read and understand.
Emailing through Hubspot is excellent as you have endless options for segmentation, personalization, and even smart content - depending on the lifecycle stage.
Forms allow you to collect tons of data on your leads and blogging is just super easy - way better than WP.
Landing pages could be better. Not easy to create without a developer. They lack ease and options that ClickFunnels or LeadPages have. We had to buy few templates (that range from $20 to $500), and it was still not the best experience.
Ultimate goal is always to generate more sales, right? HubSpot allows you to do exactly that, by following inbound strategies, creating value for prospects and generating qualified leads that you further nurture to further stages of buyer's journey.