I think my worst mods were just poor planning... I spent a lot of money on stuff I just threw away once I got a little more serious into the build. Having a solid build plan is the best mod no question!
Roush, FFRP, BMR, Steeda, Brembo, DSS, Kooks, MMD, Flowmaster, Nitto, SHR, Cervinis, SVE, McLeod... All the good shit. See my Garage for complete mod list.