I use Organteq 2 as my VST and posted a summary of how to use it in Dorico:
Hauptwerk is almost certainly the ultimate virtual pipe organ but at least for me, the expense and complication of getting it to work in Dorico are prohibitive. However, others may disagree and be able to provide help. Also, NotePerformer has organ stops available but although I use NotePerformer on a regular basis for other instruments I (perhaps foolishly) haven’t explored the organ.