My three 'can't do without' add-ons are.
1.Flight EnviromentX - which replaces default cloud, sky and water textures and animations. It transforms the atmospheric environment.
2.Horizon Simulations GenerationX Eng and Wales Photosceney - highly detailed 1m phototextures and 5m mesh
3.Aerosoft Beaver - great all purpose GA aircraft.
As Silly says it really depends on what you want to do and get out of the sim - everyone's going to be different.
All three in action.