Photographer Lisa Bettany, co-founder of the popular iPhone app Camera+, on Monday posted a series of images comparing photos taken with the iPhone 6s with its predecessors. The comparison images feature identical shots taken with each of the 9 iPhone models (sans 5c).
The image you see above shows macro photos taken of colored pencils against a white backdrop. You start to notice more details with the iPhone 3GS, and furthermore with the 4s. After that it becomes harder to discern differences, but the iPhone 6s photo clearly looks the best.