For starters get v2 mesh by Alpilot. Its great and makes a big difference. Only drawback is the long download times.
Simheaven OSM continents
http://simheaven.com/?page_id=23Simheaven Satelite scenery
http://simheaven.com/?page_id=8Carenado planes are great in X-plane.
Check out X-plane.org for individual Aiport scenery.
There are quite a few really good payware scenery's available. Check out Aerosoft and X-plane.org store.
Tom Curtis's final frontier is one of my favorites.
Another favorite is Stewart Aiport scenery.
http://beti-x.com/ Very ORBX like, even better!
If you already own a lot of FSX scenery, some of it converts very nice into X-plane with FS2XP.(free)
Some of th esceneries i have converted with ease are: Fly Tampa TNCM, Aerosoft Manhattan, Aerosoft Detroit, Aeroosft Los Angeles. There are many more, but these are the ones I remember of hand.
Hope this helps,
Rob