No theft deterrent is foolproof but the best deterrent is the one that takes the most time to bypass. Car thieves are short of time, they have to steal the car in 3 minutes or so, anything that delays that and creates a hassle for the thief is going to save your car.
Any mainstream car theft deterrent is bound to fail, because thieves come prepared for all such things. The best deterrent is one that is proprietary and catches the car thief off guard, something that he does not expect to encounter.
For eg: A proprietary fuel pump cut off switch that is activated by a combination of switches.