All of the items listed by the others before me are spot on. After some thought, I think the single best thing I have added besides the thing listed already wwould be Lower Fairings. I added some vents (Ebay) and keep them on year-round. Absolutely love them.
I'm with dw ~ LOWERS ~ then MadStad ~ Then Sanchos Wings ~
Best thing so far Cobra Power Pro Fi2000 Auto Tuner. NO MORE MAPPING. I can make any additional mods and it automatically adjusts 80 times/sec. I've always ran a power commander with good luck and was extremely skeptical, but wow what a difference, and easy to install. I've already added and taken away different things, and it always adjusts. Pricey but it was worth it. The performance blows the power commander out of the water too.
2012 FLHTRU 103"/Freedom Performance duals/Cobra power port headers/Fi2000 Power Pro tuner/18" apes/Stage II Big Sucker/13" dark smoked Madstad/Gabriel air ride
The combination of the reduced reach bars and a back rest by far. A close second is the 15" HD wind splitter windshield. I know that the bars & back rest are actually 2 items but what they do in combination with each other work as one unit to reduce back & shoulder strain. One without the other has limited effect on back & shoulder strain.
Russell Day Long seat for me. All day riding is a breeze,
Enjoy the Ride!
2013 RGU in Big Blue Pearl S&S Power Tune Duals w/V&H Slash Cuts, TTS Mastertune, Daymaker Headlights, Hawg Wired RS 400 amp, J&M Rokker speakers, SE Breather, Russell Day Long, Madstad, Progressive Monotubes, JRI Shocks, All LED lights, & several Chrome Goodies