I can't pick just one, but the AxialFlow Engineering short shifter (RPSS) and SC/AC are definately at the top of the list.

 

followed by: Tein Flex coilovers

 

JDM clear side markers on my silverstone

 

 

I don't want to seem disagreeable, but your statement is incorrect. There are many reasons for making mods, some of which have absolutely nothing to do with going faster.

I just completed my second Evo school today, and will be going for my third later this year, so I'm most certainly not against driving/track schools (in fact, back in the 70's I designed an ran a highly effective accident avoidance training program for the USAF), but when it comes to going fast, there is no substitute for track time. All the schools in the world won't help if they're not followed up with track time, and practice, practice, practice.
LOL, and besides, the schools aren't mods to the car, they're mods to the drivers brain.

 

For me: Cleared Headlights

 

Every one seems to have their own favorite mod. That's what makes modding so fun, the ability to tailor the vehicle to your tastes and make it your own.

 

I haven't done much ... but I love my interior swap

 

You wouldnt have to worry about keeping your interior cold year around if you lived in Michigan

 

Rims!