While the iPhone has been at a resolution disadvantage so far, when it comes to panoramas it matches the rest - all four cameras produce images with around 3,200px vertical resolution. Both Samsung and Apple have perfected their stitching methods and there are no stitching artifacts for static objects (moving objects can still cause issues).
The Samsung Galaxy S6 has an accurate white balance, but overexposes the image a bit, while the Galaxy S5 underdevelops the shadows. The iPhone 6's HDR functionality in the panorama mode allows it to do well both in the highlights and in the shadows.
The Galaxy Note 4 panoramas have a cold white balance (slight blue tint) while the Galaxy S5 is noticeably blue-tinted. The Apple iPhone 6 errs on the other side and produces a warm (yellowish) white balance.
The Galaxy S6 managed to keep things neutral. The Galaxy S6 also captures the most detail though images are slightly softer than the Galaxy Note 4 panoramas. The Galaxy S5 applies more sharpening than the Note, but even then the iPhone 6 retains more fine detail than it. The Galaxy S5 noise reduction smudges some of the more complex textures.