The best ruler I have ever purchases is a 18" X 6 with adjustable locking fabric Guide made by Westalee design. It has 2 screws that you adjust the wide you want, then put it next to the fabric to be cut and it holds it in place.
One that I have never use is a dresden, but then I have never made a dresden plate yet, so it may come in handy someday.
Also, I use a hand on all my rulers that can be purchases at quilt shops or in the bath supply dept. That helps hold the ruler in place, keeps finger away from the cutting area. It's worth every penny.