I would probably choose eiter the rs6 or the merc. i voted for the e55 just because it had less votes than the rs6 as of now.
I didnt choose the m5 because if i want a racer, i will get a car with less compromise to begin with. Right now i have an e30 m3, and if i had the money for one of these cars, I would have an ultima gtr in my garage too. actually, if i was given one of these, driving the merc daily and turning the e30 into race only would be a nice deal.
if i had my choice of only these cars i would get what i think would be the best ultimate street cruiser. the m5 is too common here, and like r/t said, it is just not new stuff anymore. both the rs6 and e55 are different, they stand out a bit more. they are also probably both more comfortable, and better suited for driving daily. the torque of the merc is insane.