I love my aA. The day we bought it is the last day I drove my husband's Pontiac. It is such a joy to drive, comfortable and yet it still has a bit of attitude. People are always asking if they can look inside. It is much roomier than you would think. My husband is 6'2" and has enought leg room. Even with the high price of gas we have taken more road trips this year in the xA because of the fact that we get 300 miles to a tank of gas.
I would have to say the good gas mileage, but on a more personal level it would have to be the "Frog Eye" instrument cluster.