My favorite one has to be the original PS2. I just love how it looks on top and the ridged front. It has a futuristic, monolithic feel particularly when on a vertical position.
The second has to be the PS4. The angled shape, gloss/matte finish, the reduced size... it looks unbelievably sexy.
The fat PS3 was just something else. The glossy finish all over it made it look like a luxury item.