This is way out of my league, but I'd probably go for the R8 V10. My 650xi is rare where I live, so any of the three would be equally gawked at. The R8 looks like a super car. The Maserati is very cool, but I don't think it is quite as good as the R8 V10, and the Maserati will be worthless when you go to sell it. I've never owned a Maserati or an Audi, but I'd imagine the R8 is built better and will be easier and cheaper to get serviced - may not be an issue worth factoring in, but just thought I'd throw that out there.
I haven't compared the other two to the M6 in terms of performance figures, but I think the M6 could hold its own. It is probably the best value but the least showy... and who really cares about "value" on these cars? lol
R8 is the coolest and will get the most admiring looks IMO, for what it's worth