Heavy rubber/plastic like the fishermen wear--Helly Hanson, etc. Good, tough, but plastic.
Waxed cotton like the Aussie and Brits make--Berber, Outback, etc. The ones with rain capes work, although most are lined with plain cotton that will hold water.
Ventile cotton--cotton so tight it swells solid when wet. $$$ but works like miracle fabrics claim to work.
Look at sierratradingpost.com for deals on ventile and waxed cotton.
Or you can DIY--pick a coat you like and add this:
http://gizmodo.com/neverwet-review-the-magical-water-repelling-spray-is-812797429
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