You are at the fatcat end of the market here even if you tow at 100% and the choice is limited to only a few like the premier class offerings from Merc, BMW, Lexus and the like.
Most of the reviews seem to favour the Mercedes E class and the BMW 5 series. I believe a BMW holds the UK towing speed record if you need to get to sites really fast
A dark horse might be the Chrysler 300c as the estate version gets quite a good write up.
Most if not all the heavyweights have rear wheel drive which is a plus for towing and with the sort of power levels available the choice of petrol or diesel is rather academic.
I have always fancied a BMW 7 series myself and used ones are very cheap but allowing my muddy dog on the beautiful leather seats would be sacriledge.
There's also Jags of course, an XJ or S type would manage the load well enough but they seem to be a bit low for negotiating lanes and field sites.