What is the sharpest lens you own?

Personally I find the 400 2.8 to be lacking bokeh wise. I really like the 600mm F4 L IS USM II with 2X TC for portraits on my crop body (1920mm equiv. I use a radio to tell my assistant how to pose my subjects while I'm back about 400 feet) but I wish that Canon had a 600mm f1.8 so that I can really blow the background out to smithereens at 1920mm equiv. @ 3.6 but for now 1920mm at f8 will have to settle.

If anyone couldn't tell that was a joke ;).