I've only done 4 mods to my car but...

1) Best Mod
2) Mod you wish you could take back

My best mod was probably the M5 lip spoiler that I learned about here!

 

Best mods: Tint, springs, headers, UR crank pulley & sways in that order.
Worst mod: UniChip - was over hyped over priced and under performed.

 

1) so far i'd have to say headers, but ill probly change that answer when the 3.5 comes...

2) probly the 17s i bought before the 18s, i should have went with a better wheel in the first place instead of wasting money, do it right the first time and spend the money.

 

1) supercharger

2) c/f hood .... doesnt fit quite right!

 

no bad mods so far. of course my only mods are tein basics, aem intake, hks axleback, and volks.

 

1. headers, supercharger, exhaust, 2in drop and anything else i did is great so far.

2. no bad mods yet