I have a gilet I bought about 6-7 years ago, I wear it all year round and its still going strong! Sometimes over a jersey in the summer and with jersey, arm warmers in Spring/Autumn. Been wearing it over a long sleeve jersey on winter rides. Can barely imagine riding without it now. One of the best pieces of bike clothing I ever bought.
Close 2nd goes to my Gore Bike Wear gloves, I have yet to experience cold hands while wearing them!