The only week app you don't even have to launch to use!
Week shows you the current week number on the app icon as a badge. It will update it's badge number to the current week in the background using iOS local events. From version 4.1 you can also use the built-in Notification center widget to swiftly get the current week by swiping down on the home screen.
Version 4.2 also includes an app for your Apple Watch. Now you can add week complications to your watch faces and never miss the current week again.
Inside the app you can calculate the week number for any arbitrary date just like before.
Week is also a universal app so you can run it on any of your iOS devices, like the iPod Touch, iPhone and iPad.
So if you're looking for THE week app you found it.