Most impressive:
The one that was the most impressive to look at it and hold: Hasselblad 501.
The one that had the coolest sound when fired: Hasselblad 500ELX.
The one that was amazingly smooth to operate (and produced amazing results too): Leica R8.
The one that produced the best image quality I've experienced: Mamiya RB67 ProSD (with KL lenses).
The one that is a miracle of engineering: Nikon F3HP.
Actually to qualify the last point a bit, I think the Nikon F system is simply amazing. You can buy a lens from 1970 and put it in a modern camera and it works. You can take a modern lens (ok...needs an aperture ring) and use it in a 40 year old camera. You can take a modern speedlight (say SB800 or 80DX), put it on a Auto mode, shove it on an F3 and not only you get proper exposures but even the flash indicator in the finder works. You basically have a system that ensures compatibility over 40+ years. You can even put a modern eyepiece on it.
The one that was the most impressive to look at it and hold: Hasselblad 501.
The one that had the coolest sound when fired: Hasselblad 500ELX.
The one that was amazingly smooth to operate (and produced amazing results too): Leica R8.
The one that produced the best image quality I've experienced: Mamiya RB67 ProSD (with KL lenses).
The one that is a miracle of engineering: Nikon F3HP.
Actually to qualify the last point a bit, I think the Nikon F system is simply amazing. You can buy a lens from 1970 and put it in a modern camera and it works. You can take a modern lens (ok...needs an aperture ring) and use it in a 40 year old camera. You can take a modern speedlight (say SB800 or 80DX), put it on a Auto mode, shove it on an F3 and not only you get proper exposures but even the flash indicator in the finder works. You basically have a system that ensures compatibility over 40+ years. You can even put a modern eyepiece on it.