Oh man, that's such a hard question, Yamaha Mama! That's like asking which child you like best. I love all of mine, which is why I have them. Well, OK, most of them, at any rate. And the task at hand matters too.
Just to look at, I prefer my 66K hand crank with the Lotus decals. It is so charming and reminds me of saner times.
For FMQ, my Singer 15-91 is the Queen. She reigns over her domain from a lovely Art Deco desk.
For quiet sewing and superb stitches, my 201-2 can't be beat. It reminds me of the one I learned to sew on many years ago.
For beautiful stitches and a few decorative ones, my Bernina 830 Record is always reliable.
And who could resist the red (!) Viking 6570? It never fails me when I need it, and those deco stitches are so pretty. This sweet machine was my first vintage SM.
Well, you see what I mean. Very hard to choose. Very hard. And I haven't even gotten to the portables!
PS Don't give up on your 99K -- they, too, are great little workhorses.