Congratulations on your decision to move to 6D (hope you will be as thrilled as I am, since I moved to 6D from crop, 450D).
Seeing that you have the 100mm 2.8 L macro I suggest, that you start out with that one to get some feel for the FL (not too far from 85mm as many suggested above) and the much greater bokeh on FF, than you were used to on crop. I like it for portraits - though not exactly a 'monster'.
Based on that you can make a more experienced decision.
I agree. In fact almost all Sabaki's lenses can conjure up nice bokeh in the right circumstances. Blurring isn't just the result of aperture - distance from the subject, distance of background from subject, and lens magnification all matter too (along with other factors, such as the number of aperture blades and their shape) - though of course, other things being equal, the faster the aperture the better. But other things aren't always equal - 85mm lenses tend to have a mfd of at least 3 feet, and you may well be able to get more/better background blur with a slower lens that magnifies more or lets you get closer or both (e.g. the 100L). Toss in the effect of switching to FF and it probably makes sense to suggest Sabaki doesn't buy any lenses yet - s/he may get enough blurring with what s/he already has. Of course, if one can afford an 85L, it's hard to go wrong (aside from the terrible purple fringing - scarcely better, if at all, than the 85mm 1.8 in that regard).
It may also be worth noting that the meaning of "bokeh monster" may not be clear-cut. In my experience lenses vary in bokeh appeal depending on the circumstances - I have fast lenses that create beautiful smooth blurred background effects if you can get fairly close to the subject but that, as you get further from the subject and/or the background is farther from the subject, create backgrounds that are far less smooth and even unpleasant. Some fast lenses, especially older ones, have aberrations which rather than creating a smooth blur add a distinctive character to the blur, including, in some instances, giving the effect of making out of focus highlights appear to swirl around the subject (this especially happens with lenses that create "cats' eyes" rather than smooth circles towards corners). Lenses with fewer aperture blades make out of focus highlights rapidly become less round (hexagonal, etc.) as you stop them down (a few old Russian lenses, which have up to 20 blades, never do so). Some lenses described as bokeh monsters are manual only (e.g. the Tokina 90mm 2.5 macro that's dubbed "Bokina" in some, um, circles). And so on.
So it all rather depends on the effects you want, the subjects you like to photograph and the conditions you're likely to be presented with. Which is why you might as well wait and see what you can achieve with your current lenses on your 6D when you get it; it would be a shame to spend all that money on an 85L only to discover that you can get the effects you want with your 100L....