Hi, Best bang for the Buck is Gears, but if you drive it long distances- maybe that’s not for you. A Cool Air intake, (Maybe an Airaid) and an SCT Tuner combo (American Muscle) with the “Tunes for Life” option will wake it up & allows you free Tune upgrades as you add other upgrades later.
The answer is different for everybody. It depends on what mod’s the Car has already (If anything) and how much you’ve Budgeted for this.(?)
I have completely restored my 99 v6 from scratch and ive done everything from cold air intake to bored cylinders and an upgraded camshaft. it depends on your budget really. straight pipes and 3.73 gears in the rear will give it some good zip too