Singer 15 is a favorite for quilting and free motion. The later one with the stitch length lever can drop the feed dogs. The earlier ones require a feed dog cover, they have a sort of screw and nut stitch length adjustment. I have seen people set up model 66 to use on a frame, I guess they remove the feed dogs. Some have set up 201 exclusively for free motion quilting and even embroidery. They do full size bed covers with the thicker type cotton wadding (batting?).
I am on the look for an early cast iron zigzagger, and I have read good revies on Necchi BU. You can't really go wrong with the old straight stitchers, it probably have some advantages to a lot of new machines. My favorite machine to use is a 201. The advantage with a 15 is mostly for quilting, it has the CB bobbin case many like, though it doesn't entirely exclude models with drop in bobbin case.