I have a number of mods which make me smile, and I wouldn't be able to say which is the winner because they play different parts to making the overall package, so I'll select two.
1a) ARC Air Intake
1b) Motordyne XYZ pipe
2) Eibach Pro Springs - although it gives the right look and very good handling, the amount of agro I now have to negotiate speed bumps, ramps, cats eyes the bloody lot! It's just a tad low for my V2 front. Oh and last weekend I tried to get on two ramps at the garage, to have the alignment done. Both garages couldn't get me on the ramp. So it's a flipping nuisance, but love the look.