The worst is a combination of rain and wind. That can bring on hypothermia even in the summer. Wind isn't bad as long as you're dry. Rain if unexpected and cold and heavy can make bicycling extremely dangerous in traffic. This is moreso if the rain starts out as a light rain, that's because the oild on the road make it quite slick and need a heavier or prolonged rain to wash the oils away.

Worst = Rain and wind
Almost as bad is a driving rain
A bit better is just wind or light rain
Not as bad is just wind

Cheers