Hmmmm.... I wrote an article on this very subject, some 6 years ago, for Computer Pilot magazine.
For me, the best thing was being able to create your own scenery, and Ai traffic plans.
Not around at the time, was Air Hauler - get it!
As for scenery, unfortunately the great apps from Flight1 for FSX, have not yet been updated. P3D scenery design using the few hardcore tools available, or the SDK, is a nightmare.
The one thing I look forward to the most, is for Instant Scenery, Instant Object Maker, and Airport Studio, to be updated to work in P3Dv4.
If you are still on FSX, get those..... but, expect many, late nights - it is addictive!