The $8.5 billion Delphi application - Microsoft to acquire Skype
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.