A surprisingly tough question, to pick just 1 accessory...
Probably the mirror (Mirrycle) has to be the most-used. So handy/useful and permanently ingrained in my behavior now, that when I am walking amidst people on a sidewalk, I find myself automatically glancing towards my lower-left, to check the mirror (that of course, isn't present) for people behind me before I overtake someone walking in front of me...!
In terms of most-enjoyed accessory, I really like my very compact Buckshot2.0 bluetooth speaker; with a silicon strap and the small size but good sound, I easily move it between both bikes' handlebars as needed (and sometimes even hook it on a towel rod in the bathroom for music in the shower.)
Winter-wise, the Gore jacket I have is probably my best "accessory", allowing me to comfortably commute throughout winter (well, when I had an open office building to commute to, that is, pre-teleworking!)