Best: Headlight relay harness. The halogens suck ass, but the relays help them suck less.
Worst: Dynomax cat back. I put a lot of company miles, sometimes 6 or 7 hour drives, and the exhaust note sucks for long haul. I want to listen to the radio at a lower volume. But it was still cool, especially around town.
2016 GT Premium black out 6 speed.