For BB'ing: A weight within the range that allows you to do 7 to 9 reps very quickly. Trying to shrug the shoulders as high as possible each rep. Speed is what you want, taking a tip from Olympic lifters who, for the most part, show complete trap development. From the base of the neck to the mid back area, remembering that the traps are some what diamond shaped. Also do Hi-pulls and cleans for ultimate development potential, which can be productive with DB's as well as a BB.
If you do DB shrugs seated, than that may reduce the rubbing/dragging against the outer legs each rep which standing produces. Personal view: using a trap bar, rather than a regular bar or DB, may offer better results. If using a BB, than adjust the grip to extra wide for a whole new feel to the exercise and muscles of the back. Even a closer grip (4-6 inches) can change the dynamic of the exercise.
For serious lifting: partial reps can be a thick mass builders for the traps. Any form of DL's hit the traps strongly. With DL's you are only holding the bar in place, the traps are not under a pull/contraction as they are when doing shrugs, cleans, etc.. They are getting a extreme stretch with extreme weight. These will be slower reps, for heavier reps. Throw power/hang cleans into the mix from time to time. Good Luck.