For piecing, it has to be my Featherweight. My first machine was a Janome 4800 "Quilter's edition": sadly, it wouldn't quilt! Although brilliant for piecing and quilting with a walking foot, it would never do FMQ well, though I struggled with it for several years. I didn't want another Janome after that, but after trying all the machines then available (apart from Berninas, which I can't afford) I reluctantly concluded that the 7700 was the one that felt best to me. I haven't looked back - apart from not liking to sew through thick layers (see my last post) it pieces well and quilts beautifully. I kept the 4800 because it has some decorative stitiches that aren't on the 7700, and still tend to turn to that one when I want to use a walking foot. I also have a Singer 301, which I bought when I was trying to solve the FMQ problem, and that rarely comes out of its case, so I may sell it eventually.
As someone said above, all my machines are my perfect machine - for something!