A macro lens at 1:1 and f/2.8 will give you the least DOF. What are you trying to achieve? Any lens substantially longer than 85mm (eg, 300mm f/2.8) will have much less DOF than the 85L. But the perspective will be so different that there's not much point comparing them.

Word to the wise... DOF taken to the extreme is just kind of boring. You can completely obliterate the background to the point where all you see is a head floating against a field of color, but what's the point? Might as well shoot against a sweep.

How about a nikon 105 macro f2.8 with 2x teleconverter? Im using it quite sometime now and very effective.