Upgraded my front suspension.
Remember that old saying "you don't know what you don't know"?
I've always upgraded my HD's rear suspensions to non-air shocks (usually Progressive) and considered it good to go on my touring bikes. Ride was noticeably improved, especially 2-up. Never gave a thought to my front suspension thinking it was good enough for the manner in which I ride until I read all the good reviews of the 2014 models and how others have upgraded their pre-2014 bikes.
I recently had the HD premium front fork suspension kit installed on my RGU and am now kicking myself for not doing it 3 years ago. In the past local roads that have developed potholes and been filled in over and over again have always felt "jarring" to me as the sensation of riding over them was transmitted thru the handle bars. Now with the upgraded front suspension that jarring sensation is gone, replaced by the feeling of going over small bumps (best way I can describe it) on those same roads. Cornering feels firmer and more controllable also.
For me, the rear suspension upgrade represents 40 percent of the equation and the front is 60 percent. Glad I spent the money on the RGU front upgrade. Now I'm going to do the same to my SG.