WARNING, OUTDATED ANSWER: this answer is not up to date as per WWDC 2015, for the correct answer refer to the accepted answer (Daniel Hall) above. This answer will stay for record.

Summarized from the developer library:

From a practical perspective, in iOS and OS X outlets should be defined as declared properties. Outlets should generally be weak, except for those from File’s Owner to top-level objects in a nib file (or, in iOS, a storyboard scene) which should be strong. Outlets that you create will therefore typically be weak by default, because:

• Outlets that you create to, for example, subviews of a view controller’s view or a window controller’s window, are arbitrary references between objects that do not imply ownership.

• The strong outlets are frequently specified by framework classes (for example, UIViewController’s view outlet, or NSWindowController’s window outlet).

  @property (weak) IBOutlet MyView *viewContainerSubview;
  @property (strong) IBOutlet MyOtherClass *topLevelObject;
  

The current recommended best practice from Apple is for IBOutlets to be strong unless weak is specifically needed to avoid a retain cycle. As Johannes mentioned above, this was commented on in the “Implementing UI Designs in Interface Builder” session from WWDC 2015 where an Apple Engineer said:

And the last option I want to point out is the storage type, which can
either be strong or weak. In general you should make your outlet
strong, especially if you are connecting an outlet to a subview or to
a constraint that’s not always going to be retained by the view
hierarchy. The only time you really need to make an outlet weak is if
you have a custom view that references something back up the view
hierarchy and in general that’s not recommended.

I asked about this on Twitter to an engineer on the IB team and he confirmed that strong should be the default and that the developer docs are being updated.

https://twitter.com/_danielhall/status/620716996326350848
https://twitter.com/_danielhall/status/620717252216623104

While the documentation recommends using weak on properties for subviews, since iOS 6 it seems to be fine to use strong (the default ownership qualifier) instead. That’s caused by the change in UIViewController that views are not unloaded anymore.

• Before iOS 6, if you kept strong links to subviews of the controller’s view around, if the view controller’s main view got unloaded, those would hold onto the subviews as long as the view controller is around.
• Since iOS 6, views are not unloaded anymore, but loaded once and then stick around as long as their controller is there. So strong properties won’t matter. They also won’t create strong reference cycles, since they point down the strong reference graph.

That said, I am torn between using

@property (nonatomic, weak) IBOutlet UIButton *button;

and

@property (nonatomic) IBOutlet UIButton *button;

in iOS 6 and after:

• Using weak clearly states that the controller doesn’t want ownership of the button.

• But omitting weak doesn’t hurt in iOS 6 without view unloading, and is shorter. Some may point out that is also faster, but I have yet to encounter an app that is too slow because of weak IBOutlets.

• Not using weak may be perceived as an error.

Bottom line: Since iOS 6 we can’t get this wrong anymore as long as we don’t use view unloading. Time to party. 😉

I don’t see any problem with that. Pre-ARC, I’ve always made my IBOutlets assign, as they’re already retained by their superviews. If you make them weak, you shouldn’t have to nil them out in viewDidUnload, as you point out.

One caveat: You can support iOS 4.x in an ARC project, but if you do, you can’t use weak, so you’d have to make them assign, in which case you’d still want to nil the reference in viewDidUnload to avoid a dangling pointer. Here’s an example of a dangling pointer bug I’ve experienced:

A UIViewController has a UITextField for zip code. It uses CLLocationManager to reverse geocode the user’s location and set the zip code. Here’s the delegate callback:

-(void)locationManager:(CLLocationManager *)manager
   didUpdateToLocation:(CLLocation *)newLocation
          fromLocation:(CLLocation *)oldLocation {
    Class geocoderClass = NSClassFromString(@"CLGeocoder");
    if (geocoderClass && IsEmpty(self.zip.text)) {
        id geocoder = [[geocoderClass alloc] init];
        [geocoder reverseGeocodeLocation:newLocation completionHandler:^(NSArray *placemarks, NSError *error) {
            if (self.zip && IsEmpty(self.zip.text)) {
                self.zip.text = [[placemarks objectAtIndex:0] postalCode];
            }
        }];    
    }
    [self.locationManager stopUpdatingLocation];
}

I found that if I dismissed this view at the right time and didn’t nil self.zip in viewDidUnload, the delegate callback could throw a bad access exception on self.zip.text.

IBOutlet should be strong, for performance reason. See Storyboard Reference, Strong IBOutlet, Scene Dock in iOS 9

As explained in this paragraph, the outlets to subviews of the view
controller’s view can be weak, because these subviews are already
owned by the top-level object of the nib file. However, when an Outlet
is defined as a weak pointer and the pointer is set, ARC calls the
runtime function:

id objc_storeWeak(id *object, id value);

This adds the pointer
(object) to a table using the object value as a key. This table is
referred to as the weak table. ARC uses this table to store all the
weak pointers of your application. Now, when the object value is
deallocated, ARC will iterate over the weak table and set the weak
reference to nil. Alternatively, ARC can call:

void objc_destroyWeak(id * object)

Then, the object is
unregistered and objc_destroyWeak calls again:

objc_storeWeak(id *object, nil)

This book-keeping associated
with a weak reference can take 2–3 times longer over the release of a
strong reference. So, a weak reference introduces an overhead for the
runtime that you can avoid by simply defining outlets as strong.

As of Xcode 7, it suggests strong

If you watch WWDC 2015 session 407 Implementing UI Designs in Interface Builder, it suggests (transcript from http://asciiwwdc.com/2015/sessions/407)

And the last option I want to point out is the storage type, which can either be strong or weak.

In general you should make your outlet strong, especially if you are connecting an outlet to a sub view or to a constraint that’s not always going to be retained by the view hierarchy.

The only time you really need to make an outlet weak is if you have a custom view that references something back up the view hierarchy and in general that’s not recommended.

So I’m going to choose strong and I will click connect which will generate my outlet.

In iOS development NIB loading is a little bit different from Mac development.

In Mac development an IBOutlet is usually a weak reference: if you have a subclass of NSViewController only the top-level view will be retained and when you dealloc the controller all its subviews and outlets are freed automatically.

UiViewController use Key Value Coding to set the outlets using strong references. So when you dealloc your UIViewController, the top view will automatically deallocated, but you must also deallocate all its outlets in the dealloc method.

In this post from the Big Nerd Ranch, they cover this topic and also explain why using a strong reference in IBOutlet is not a good choice (even if it is recommended by Apple in this case).

One thing I wish to point out here, and that is, despite what the Apple engineers have stated in their own WWDC 2015 video here:

https://developer.apple.com/videos/play/wwdc2015/407/

Apple keeps changing their mind on the subject, which tells us that there is no single right answer to this question. To show that even Apple engineers are split on this subject, take a look at Apple’s most recent
sample code, and you’ll see some people use weak, and some don’t.

This Apple Pay example uses weak:
https://developer.apple.com/library/ios/samplecode/Emporium/Listings/Emporium_ProductTableViewController_swift.html#//apple_ref/doc/uid/TP40016175-Emporium_ProductTableViewController_swift-DontLinkElementID_8

As does this picture-in-picture example:
https://developer.apple.com/library/ios/samplecode/AVFoundationPiPPlayer/Listings/AVFoundationPiPPlayer_PlayerViewController_swift.html#//apple_ref/doc/uid/TP40016166-AVFoundationPiPPlayer_PlayerViewController_swift-DontLinkElementID_4

As does the Lister example:
https://developer.apple.com/library/ios/samplecode/Lister/Listings/Lister_ListCell_swift.html#//apple_ref/doc/uid/TP40014701-Lister_ListCell_swift-DontLinkElementID_57

As does the Core Location example:
https://developer.apple.com/library/ios/samplecode/PotLoc/Listings/Potloc_PotlocViewController_swift.html#//apple_ref/doc/uid/TP40016176-Potloc_PotlocViewController_swift-DontLinkElementID_6

As does the view controller previewing example:
https://developer.apple.com/library/ios/samplecode/ViewControllerPreviews/Listings/Projects_PreviewUsingDelegate_PreviewUsingDelegate_DetailViewController_swift.html#//apple_ref/doc/uid/TP40016546-Projects_PreviewUsingDelegate_PreviewUsingDelegate_DetailViewController_swift-DontLinkElementID_5

As does the HomeKit example:
https://developer.apple.com/library/ios/samplecode/HomeKitCatalog/Listings/HMCatalog_Homes_Action_Sets_ActionSetViewController_swift.html#//apple_ref/doc/uid/TP40015048-HMCatalog_Homes_Action_Sets_ActionSetViewController_swift-DontLinkElementID_23

All those are fully updated for iOS 9, and all use weak outlets. From this we learn that A. The issue is not as simple as some people make it out to be. B. Apple has changed their mind repeatedly, and C. You can use whatever makes you happy 🙂

Special thanks to Paul Hudson (author of www.hackingwithsift.com) who gave me the clarification, and references for this answer.

I hope this clarifies the subject a bit better!

Take care.

From WWDC 2015 there is a session on Implementing UI Designs in Interface Builder. Around the 32min mark he says that you always want to make your @IBOutlet strong.

Be aware, IBOutletCollection should be @property (strong, nonatomic).

It looks like something has changed over the years and now Apple recommends to use strong in general. The evidence on their WWDC session is in session 407 – Implementing UI Designs in Interface Builder and starts at 32:30. My note from what he says is (almost, if not exactly, quoting him):

• outlet connections in general should be strong especially if we connect a subview or constraint that is not always retained by the
  view hierarchy

• weak outlet connection might be needed when creating custom views that has some reference to something back up in the view hierarchy
  and in general it is not recommended

In other wards it should be always strong now as long as some of our custom view doesn’t create a retain cycle with some of the view up in the view hierarchy

EDIT :

Some may ask the question. Does keeping it with a strong reference doesn’t create a retain cycle as the root view controller and the owning view keeps the reference to it? Or why that changed happened?
I think the answer is earlier in this talk when they describe how the nibs are created from the xib. There is a separate nib created for a VC and for the view. I think this might be the reason why they change the recommendations. Still it would be nice to get a deeper explanation from Apple.

I think that most important information is:
Elements in xib are automatically in subviews of view. Subviews is NSArray. NSArray owns it’s elements. etc have strong pointers on them. So in most cases you don’t want to create another strong pointer (IBOutlet)

And with ARC you don’t need to do anything in viewDidUnload