The Buick is just a cheap way of getting points. Its not a drift car. Besides, until PD implements an online Drift Trial mode outside of the seasonal events, you're better off going into online free-run lobbies or practice mode.
The physics are vastly different as well... so don't expect the same results in a lobby as what you're used to getting in Drift Trials.
The "best drift car" is not based on points awarded for drifting the racing lines at 45 degree angles. The best drift car is the car that you can control comfortably and consistently around multiple tracks of various turns and layouts at every speed in which your car is able to slide.