The answer of course is the 400 2.8 IS. In terms of bokeh, image quality, color and contrast nothing else can beat it.
100-400, nice but no 2.8 bokeh, not the sharpest lens but still pretty good for a zoom
70-200 with 2x, ehh.. 2x really kills this lens, works pretty nice with a 1.4x though
300 f4 with 1.4x, pretty good, but the TC is still degrading things, bokeh isnt as nice as a 2.8 either
400 5.6, nice but its 5.6 so your missing out on some bokeh and the faster lens can of course be stopped down where they really shine
400 f4 DO, havent used in person but its just simply not "the best" good if you need the light weight with decent speed though. My pick if I only shot wildlife
300 2.8 with 1.4x, still pretty sharp, loses very little with TC but the bokeh isnt as nice due to the TC
So there you go....
Of course keep in mind that each of these lens all have their own given strengths and those are usually worth a good bit more than just the image quality alone