The best sewing machine is really based on your budget of how much you are will to spend. Singer sewing machines are the best machine for beginner dressmakers that cost around $200. Simple, easy to handle inexpensive. The professional models can cost $1500 an up.
The best sewing machine for a dressmaker is the Nikon Singer Confidence. I personally own one and it is a very good machine. There are lots of stiching options and the machine is very simple to use.
The Brother CS6000i computerized sewing machine is the best sewing machine if you are looking to sew dresses. It is good for people at any level of sewing also.
The machine,depending on how accomplished a dressmaker you might be, could be a very basic machine for the beginner, or a more complicated one for an advanced seamstress.