Surely the A4000T has to be least popular, on the grounds that there weren't many of them made...
Failing that, the A600 would be least popular because they're silly. My first Amiga was a brand new A600 (upgraded from C64) back in 1992. I remember thinking I would have been better off getting an old A500.
If the A600 had stuck with it's original name - the A300, I believe it would have made a lot more sense. Damn the upper echelons of Commodore mis-management!!
