The correct location is APPDATA if the files that need to be stored are different for each user or ProgramData if they are the same for all users. If you need your users to brwose the folder directly you can create folder shortcut next to your application shortcut, so you don't need to explain them how to make the folder visible on systems where it is hidden by default.
@Feco It is incorrect to store application files into Program Files folder, this is location that requires administrative privileges to write in, so NOT all users can write there. In Program Files folders you should only install your application binaries, if the application needs to be installed per-machine. If the application should be installed per-user, then APPDATA folder is the one.
Also, installing applications directly on "C:\" (Windows partition root) is an old habit, not standard practice. This has not advantages compared to installing the app in Program Files and just leads to cluttering your file system with various application folders on "C:\".