First was CD lights. Dealer threw them on with the purchase of the bike. My add on list is extensive. Better question would be favorite, or most effective/favorite mods.
My first mod was to my a$$, had to get it stitched up after the cvo purchase! LoL, nah, but like others have said, lights have been my first mod on my last 2 bikes, cvo came with led's but I still replaced them with custom dynamics pro beams.
Before my RK left the dealer I got them to install the front fork fangs and heated grips then remove the heal shifter. My first mods at home was switching to all LED lighting.
Normally I do air cleaner, exhaust, and a dyno tune, then bars. On this new one I'm upgrading the speakers and installing an amp first. Then it's a little taller windshield and sissy bar setup for the wife. Fix the creature comforts first.