On my '95ATX, as much as I'd like to say it was the BAT suspension, I have to say it was upgrading to the SVT exhaust. I loved the sound of it, even hamstrung by the auto tranny.
On my '97SE, so far it's been wiring the Pioneer with an aux line in so I can get the audio output from my PDA (navigation & music) through my stereo. That has been so useful. But I have an SVT exhaust in my garage waiting to go on this car, so my "best mod" may change soon.
