So if you had Photoshop only on the Mac, say, and the rest of the platforms had stuff like MS Paint, you'd still not consider it the best of its kind?

Professionals that are not system administrators don't switch between platforms that often. A software can very well be the best of its kind and single platform.

Although in my experience, the best software of its kind is usually limited to one platform. Otherwise, you're stuck catering to the lowest common denominator of all supported platforms.