Surely this is an easy enough one... it has to be about the hardware platform. Amiga started with a very advanced and interesting set up at the hardware level, which took the x86 machines about 10? years to really catch up with and then pretty much overtake. At the heart was the Motorola 680x0 series of CPUs. As far as I can follow logically, the last "real" Amiga has to be the A4000 as it was actually released by Amiga as a "successor" to the previous Amiga machine. The OS is irrelevant in as much as "Amiga" was a hardware manufacturer, or rather "Amiga" was a name for a piece of hardware. Whereas "Windows", "Linux" & "AmigaOS" etc are names of software. Albeit pretty critical software, but you could actually write your own OS or simply enter machine code.... not recommended though :¬)
edit: mispelt 'heart'