I have always been partial to the black Singer 15-91s. Gear drive, no belts to replace, simple straight stitch, easy to maintain, pretty much bullet-proof in every way.
But now that I have picked up a 15-125 (and a 185K), I may have to rethink my fondness for black. I don't know why I like these old green Singer machines, but they just look so cool.
And there are other green machines, not just Singer.
New Home 270
Adler 153A (with optional fully-deteriorated timing belt, makes it pretty much useless except for occasional use as a doorstop)
I guess it was the 'in' thing in late 50s to offer a green sewing machine.