Whatever you can afford. Are you looking at 2D or 3D?
There are quite a few free apps. BRLCAD, Draftsight, FreeCAD to name a few off the top of my head. Some 2D, some 3D also available for reasonably low cost, RhinoCAD
Deeper pockets SolidWorks, Inventor, ProEng, Catia.
Can't think of any more at the moment.