NASCARs - I would take the Gen6 Fusion, the styling is fantastic. I know nothing about these cars though, so my guess is as good as yours.
Supercars from around the World - As soon as I even glanced at this comparison, I thought the GT and LFA would be my two favourites, and they are. I really dislike the Carrera as a car, the 458 is the Spider model (yuck) and the Vanquish is gorgeous as ever, but I just can't imagine picking it over an LFA or GT. The LFA and GT are both beautiful cars, they both drive fantastically, they have both been intensely developed by two of the best (in my opinion) car manufacturers of all time. As much as it pains me, I would have to choose the GT, as I just can't resist the thrill of an engine such as that. The LFA is still one of my dream cars, but the GT just slightly shaves it and gets the win.
Riviera vs Montego - I don't see why anyone would pick the Montego; it seems like it's trying so hard to be a Mustang, but failing oh so badly. I would much rather have the more original Buick Riviera. The styling on that Buick is just so well designed and it still looks cool, even after all these years. I would gladly drive it around London and even show my friends, which is something that I almost never do with cars that are that old.