When I worked for the Cooperative Extension Service, I learned to teach sewing machine maintenance. The first time though that I gave a demonstration on how to completely clean and oil a Featherweight, I couldn't get it back together correctly! Well, I quickly learned and really enjoyed using machines I could "fiddle" with. All of this experience was very helpful when I started teaching at the high school and found 5 brands of sewing machines in the department. We were able to keep them all working well beyond their expected life span.
At home, I regularly use three old Vikings, one new Viking, my Featherweight, an old green Elna I got at a yard sale and 4 different sergers. I also have two Singers from the 50's, a 1922 Singer, two treadle machines and a Bernette 600 I purchased recently from Craig's list. Needless to say, I've had to rearrange my sewing room so I can set up enough tables.
It is really helpful though to have a variety of machines because they all handle a little differently and I can have them set up with different color threads, needles or throat plates (sergers). I've also given away several machines to my daughter and niece over the years.
One machine I've never used is a commercial sewing machine. The neighbor lady donated one to our department and before I ever learned to use it, I retired. I in turn, donated it to another school and she has used it.
Probably the machine I treasure the most is the White treadle that was my Grandmother's. She bought it used so she could make a living as a dressmaker. She sewed on it up until she was in her 90's. I can't even get near the quality she produced, but I keep trying. I do have some of her quilt tops that I want to finish someday and I have a few other things she did, along with the sewing machine. I've never used it, but my Sister-in-law did a few years ago, and it worked fine.
My guess as to the sewing machine I would LEAST want is a Bernina. It is a wonderful machine, but in my experience it doesn't allow me enough flexibility in changing tension and such. Here are some pictures.