For me it was Mac OS X. The reason I went with the Amiga in the first place (in 1992 or so) was that my Sinclair QL was getting a bit dated, I was tired of writing all of the software I needed completely by myself, and I couldn't move from something with clean 32 bit programming and preemptive multitasking to PC-DOS / Windows or Mac OS. Plus, I had work buy an Amiga since it could also be used to run Mac software.
When Mac OS X came out, I had a real GUI, a good programming environment, preemptive multitasking, protected memory and lots of applications. No, GNU/Linux or BSD with KDE or Gnome wouldn't do - they suck as GUIs.
