It depends on what the data is. If the data must be available to ALL users i.e. it is for the application regardless of the user then use;
Environment.SpecialFolder.CommonApplicationData
If the data is specific to a single user then use;
Environment.SpecialFolder.ApplicationData
OR
Environment.SpecialFolder.LocalApplicationData
You use
Environment.SpecialFolder.ApplicationData
for specific user data as a general rule because data stored here is also replicated if roaming profiles are used.