Great thread - so many great ideas and definitely noticing some trends. I wish the OP had requested everyones top 5 mods!
Since everyone is naming the obvious ones (seats, handlebars, windshields) I won't mention those although the Harley Hammock seat and Wind Vest 8" are winners for me, and I'll go a different route that I didn't see mentioned.
Something so simple, relatively inexpensive (although ridiculously expensive for what it is!), and easy to install but made a huge difference for me is the kickstand extender. I have short legs so getting that side stand out and even worse, back, was a pain in the a$$. This little mod gives me just enough extra that deploying and retrieving the kickstand is now a snap.
A mod that I'm dying to do that will probably be the best one yet for me (and is definitely supported by the vast majority here) is adding Heritage bars. That's next!
2013 Road Glide Custom CVO in Roman Gold/Burnt Emerald, Le Pera Maverick, Vance & Hines Power Duals, SERT, MadStad 13", Kury ISO grips & Scythe mirrors.