Visibility above any other consideration, with multiple flash patterns. I personally like the light bar variant - all my blinky lights are Cygolight Hot Rods. Row of extremely bright LED's, about 2". Seven (?) blinking patterns from basic strobe to "Be seen from the Space Station" mode. Mounted properly they are visible from a long way off - I mount them on the left side seat stay and on the right fork blade. Mounting that way angles them up slightly and puts them directly in the view of an approaching motorist.
