One of my favorite applications built with Delphi is Skype for Windows. We use Skype here every day. It's been great to see the success of Skype and so many millions of users experiencing and relying on a Delphi application for their day to day communication.
Today Microsoft announced that they will be acquiring Skype for $8.5 billion. I think that makes Skype the most valuable Delphi application out there.
Of course there are also many other great applications built with Delphi and you can see some of them in the Embarcadero Delphi Application Showcase.
If you haven't submitted your Delphi application for the showcase yet, you can do that here. We're always happy to learn about what you're creating with Delphi.