I cant really decide what any one car of the decade can be as there are wayyy too many variables for such a specific statement. I will say that the bugatti veyron was a feat of engineering and the fact that it actually cost more to produce than they sold it for says that pretty well.

Best american bang for the buck and by this I mean domestic made would be the c6 z06 hands down or the zr1 if you are talking about bang for alotta buck

Japanese would be the gt-r even though the transmissions do explode and fluid for the tranny's cost over 100 a quart.

ACR is the best track destroyer but I dont know about dealing with it everyday and to be honest I have a soft spot for chevy's and lsx's.

Still love the zonda r and koeniggsegggggkljaf;lja;jdfaj;f ccxr or however you spell it too along with the fxx and enzo.