Starting from iOS 6 right way to do it by using SKStoreProductViewController class.
Swift 5.x:
func openStoreProductWithiTunesItemIdentifier(_ identifier: String) {
    let storeViewController = SKStoreProductViewController()
    storeViewController.delegate = self
    let parameters = [ SKStoreProductParameterITunesItemIdentifier : identifier]
    storeViewController.loadProduct(withParameters: parameters) { [weak self] (loaded, error) -> Void in
        if loaded {
            // Parent class of self is UIViewContorller
            self?.present(storeViewController, animated: true, completion: nil)
        }
    }
}
private func productViewControllerDidFinish(viewController: SKStoreProductViewController) {
    viewController.dismiss(animated: true, completion: nil)
}
// Usage:
openStoreProductWithiTunesItemIdentifier("1234567")
Swift 3.x:
func openStoreProductWithiTunesItemIdentifier(identifier: String) {
    let storeViewController = SKStoreProductViewController()
    storeViewController.delegate = self
    let parameters = [ SKStoreProductParameterITunesItemIdentifier : identifier]
    storeViewController.loadProduct(withParameters: parameters) { [weak self] (loaded, error) -> Void in
        if loaded {
            // Parent class of self is UIViewContorller
            self?.present(storeViewController, animated: true, completion: nil)
        }
    }
}
func productViewControllerDidFinish(_ viewController: SKStoreProductViewController) {
    viewController.dismiss(animated: true, completion: nil)
}
// Usage:
openStoreProductWithiTunesItemIdentifier(identifier: "13432")
You can get the app's itunes item identifier like this: (instead of a static one)
Swift 3.2
var appID: String = infoDictionary["CFBundleIdentifier"]
var url = URL(string: "http://itunes.apple.com/lookup?bundleId=\(appID)")
var data = Data(contentsOf: url!)
var lookup = try? JSONSerialization.jsonObject(with: data!, options: []) as? [AnyHashable: Any]
var appITunesItemIdentifier = lookup["results"][0]["trackId"] as? String
openStoreProductViewController(withITunesItemIdentifier: Int(appITunesItemIdentifier!) ?? 0)
Swift 2.x:
func openStoreProductWithiTunesItemIdentifier(identifier: String) {
    let storeViewController = SKStoreProductViewController()
    storeViewController.delegate = self
    let parameters = [ SKStoreProductParameterITunesItemIdentifier : identifier]
    storeViewController.loadProductWithParameters(parameters) { [weak self] (loaded, error) -> Void in
        if loaded {
            // Parent class of self is UIViewContorller
            self?.presentViewController(storeViewController, animated: true, completion: nil)
        }
    }
}
func productViewControllerDidFinish(viewController: SKStoreProductViewController) {
    viewController.dismissViewControllerAnimated(true, completion: nil)
}
// Usage
openStoreProductWithiTunesItemIdentifier("2321354")
objective-C:
static NSInteger const kAppITunesItemIdentifier = 324684580;
[self openStoreProductViewControllerWithITunesItemIdentifier:kAppITunesItemIdentifier];
- (void)openStoreProductViewControllerWithITunesItemIdentifier:(NSInteger)iTunesItemIdentifier {
    SKStoreProductViewController *storeViewController = [[SKStoreProductViewController alloc] init];
    storeViewController.delegate = self;
    NSNumber *identifier = [NSNumber numberWithInteger:iTunesItemIdentifier];
    NSDictionary *parameters = @{ SKStoreProductParameterITunesItemIdentifier:identifier };
    UIViewController *viewController = self.window.rootViewController;
    [storeViewController loadProductWithParameters:parameters
                                   completionBlock:^(BOOL result, NSError *error) {
                                       if (result)
                                           [viewController presentViewController:storeViewController
                                                              animated:YES
                                                            completion:nil];
                                       else NSLog(@"SKStoreProductViewController: %@", error);
                                   }];
    [storeViewController release];
}
#pragma mark - SKStoreProductViewControllerDelegate
- (void)productViewControllerDidFinish:(SKStoreProductViewController *)viewController {
    [viewController dismissViewControllerAnimated:YES completion:nil];
}
You can get kAppITunesItemIdentifier (app's itunes item identifier) like this: (instead of a static one)
NSDictionary* infoDictionary = [[NSBundle mainBundle] infoDictionary];
    NSString* appID = infoDictionary[@"CFBundleIdentifier"];
    NSURL* url = [NSURL URLWithString:[NSString stringWithFormat:@"http://itunes.apple.com/lookup?bundleId=%@", appID]];
    NSData* data = [NSData dataWithContentsOfURL:url];
    NSDictionary* lookup = [NSJSONSerialization JSONObjectWithData:data options:0 error:nil];
    NSString * appITunesItemIdentifier =  lookup[@"results"][0][@"trackId"]; 
    [self openStoreProductViewControllerWithITunesItemIdentifier:[appITunesItemIdentifier intValue]];