Name the single best mod you've done to your car, past or present,
I'll start

Car - E39 M5
Mod - oem 545i short shifter and weighted knob
Cost - about 120e


I done a simular mod to my old 330, namely the UUC setup and although the UUC setup is IMO far better to the 545i setup it's about 5 times the price so bang for buck I'd choose the oem setup