Regular bikes: front/rear lights. Usually leave the reflectors on the back,will leave the front on if I don't need the bar space. Spoke reflectors I'll leave on,but won't replce them if they fall off. Reflective sidewall tires if I can.

Bad weather bike: front headlight and blinky,2 rear lights,reflective tape on headtube/seatstays/left chainstay,reflective sidewall tires.

I always try to get my tires with reflective sidewalls. Two big reflective hoops that can be seen from a very wide angle.