Helix Euro Rear Taillight Assemblies
Probably the first "major" mod to my GTI is the Euro taillights. It took me about 5 hours to install the taillights, including coding. Removing all the trim in the hatch was the first struggle, since this was the first time it was done and the metal detents were hard to give way. Eventually everything gave way and I got all the trim out. After installing the inner and outer taillights, and performing the coding, the lights still weren't working right, which ended up being a minor wiring error I noticed a few days later. All in all, this is probably the best mod I could see doing to the car. It improves the appearance 10 fold, and there's no doubt whether your brake lights are working or not.
Amber turn signals are great. The reverse lights now take a H21W bulb as well.