What is an Amiga is a little like asking which women are hot. There are some that we can pretty much all agree on, but as you get farther out, everybody starts getting their own ideas. Some like big tits, and can live with a few extra pounds elsewhere. Some want them thin, and don't care how flat she is. Some just look at the face, and others just look at the butt. That is the boat we are in with Amigas. We can all agree that the A500, A1000, A2000, A3000 are real Amigas, but after that we are not necessarily looking at the same parts when we want to add more units to the list.
I like to be pragmatic. Hot chicks are the ones that will sleep with me, and Amigas are systems that can run a reasonably large selection of the software released for the A500, A1000, A2000 and/or A3000. After that, the various traits define which is the best, not which is an Amiga.