I don't know if it's bunk advice but my Grandfather who was in WW2 always told me to keep my chest dry in the rain. If your chest gets wet for long periods that's when you will get sick. So most importantly for me is a good waterproof jacket. I recently went to buy a Helly Hanson but I was surprised! Despite claiming to be waterproof none of the zips on the pockets etc actually were.

I think though that my classic bikes have turned me into a fair weather rider. I spend hours cleaning and polishing so I don't want to get it wet and have to start all over again.