as much as I like the 2010 maro for the amount of notoriety it will gain your shop, the CTS-V will/should win out. They are simply baller-tastic!
A genesis V8 would also be nice, but from a business aspect, you will want a powerful vehicle that will offer drool-worthy appeal, but at the same time, move up your level of clientele. That being said, I'd recommend you do a g37.
ninja edit....I'd also like to add M5 to the list. I'd love to see one of those with forced induction. I think it would bump your clientele up a few notches, retain the ballerific status like the CTS-V and be just exotic enough to be nice.