You don't want to know which diff is best, you want to know what gear ratio
is the best.

Higher numbers will give greater accelleration, but lower top end. For
example, the stock ratio is going to something in the range or 3.45:1,
4.10:1 will give better acceleration, 4.56:1 will be even better, but the
top speed will suffer because the engine will top out earlier.

A car turning 3.45 gears will go faster than a car spinning 4.56s, but it
will not be as quick. You seem to be interested in quickness at the expense
of top end, which is why I went off on this particular tangent. That is,
assuming you have enough horsepower to allow the engine to reach peak RPMs
in the highest gear, the speed at that point will be greater with taller
gears (lower numeric value) in the differential than with lower gears
(higher numeric value).

I used these numbers ONLY to illustrate the point, I have no clue as to the
availability of them for a 7 Series car, the acutal ratio you get will
depend on physical size factors of the differential housing itself.

Here's a place to go shopping:  http://www.koalamotorsport.com/

While it's nice to improve the acceleration of your car, a higher ratio
differential will make it's presence known most often with higher engine RPM
at cruising speeds.  It might not be particularly rewarding to cruise the
freeway at 4000 RPM.

R / John