Well Hacksaw, there is no "best". I bet most here use a gaggle of different programs to achieve their goals. If your goals are to make things that have basic geometric shapes, a decent 2d (or 3d) Cad program will be a must. The good news there is you can find that inexpensively. Myself, I'd take Autosketch or Punchcad (both under $100) over TurboCad any day when it comes to being inexpensive yet fully, fully capable.
If your hoping to do things that are more artistic, then you may try an old version of Corel Draw (payware), or you can do pretty well with the freeware vector programs like OpenOffice Draw or Inkscape.
In either case above, you will essentially need to export your drawings into a program that will create your G-Code to run the machine. This is typically referred to as a Cam program. You should be able to use a DXF to G-code converter, but it can get difficult depending on what you want to make. True, low budget Cam programs exist out there... Sheetcam is one on the top of my head for 2.5D.... I think Inkscape has a G-code add-on for free. Of course, sometimes free just means user headaches.
For a little bit of hard earned cash, you probably just can't beat purchasing VCarve from Vectric (or Aspire). You can import all sorts of stuff into it, you can draw your basic geometric shapes in it, and you can create artistic work in it. The best part is you can also create your G-code in it that's ready to send to your machine.
Above all that stuff you find the true "Cad/Cam" programs, which start around the $1000 mark and work their way up to many thousands of dollars per user. "Best" really doesn't exist. What various people call best (for them) can be found in arguments all over the forums.