Maybe I have used 'best modification' elsewhere, I am not sure. I do have to clarify that the one I am about to show you probably does take the cake....
I mention this already in my build log. But its to do with the upper dash panel. The stock Jeep one is just a pice of smooth plastic, which prevents the storage of small items up there. While I thought this may have been done for passenger safety (TJ had this functionality) it also returned with the JK dash redesign in 2011.
To remedy this in the earlier Jeeps I highly recommend the Daystar dash panel, the replacement panel simply adds some places to store small items. It has also proven to also be a great base for further modifications and tweaks. To date I have modified it in 4 unique ways.
The first thing I have done with mine was to spray it with plasti-dip (or similar product) this I feel adds a slightly grippier texture to prevent your items sliding.
The second thing I have done was add some "RAM" mounts to both sides. This system of balls, arms, and holders etc works phenomenally and is extremely well built. The downside is the price, I probably have over $100 of components on mine. Its nice to think that is modular and the arms and device holders can be easily transferred to another vehicle. Another upside is I can quickly switch between two phone mounts and replace one with a go pro etc. In the future I also plan to have a small tablet, I will then only need the new cradle or "X-mount" and I can swap it into the mix in seconds. My most common use is my phone closest to me and my old iPhone running MotionX GPS on the second.
The third modification was to add some angle aluminum and a fin to the back. I did this to help direct more defrost air in front of the driver. I am really disappointed that Jeep would have designed this air to come out the centre in a fan pattern. Several times each winter I will be driving in cold enough temperatures where the corners fog up and/or ice builds up when snow from the centre melts and refreezes on the edges. Unfortunately I cannot honestly say that I notice this modification has done anything to help this I will keep tweaking it trying to make it better.
The last change I made is stealthier than some of the others. I have drilled two small holes for LEDS in either corner. I wanted to work on adding red LED interior lighting for when the passenger needs to find something or view a map while driving. This is currently the first and only place I have installed these I do have some LED strip lights that I will be mounting under the dash as well as under the different tiers of the centre stack.
My future modifications may be as simple as adding a couple holes in either side to loop some black zap straps through this way I can route wires to my devices keeping it out of the drivers way.