First thing you'll want isFS Global 2010. That will increase the detail is the points of elevation making land look more realistic. Next, I would get Real Environment Extreme. REX is an excellent programs which gives you hi-def textures for everything from runways to clouds. My Traffic X adds many different kinds of AI aircraft to the world around you.
Aside from those 3, There are some smaller mods which can give you photo-realistic scenery for cities or just very high resolution data for a particular region(like the Pacific Northwest). My favorite region mods are from FTX. There are also many, many airplanes, helicopters, and missions you can add, but those are all personal preference.
Anything else you want to know?
ETA: MegaSceneryX also has some really good scenery.