I've got Toggi Extreme - super to ride in and stayed waterproof through most of the last winter's downpours. They don't stay dry in torrential rain though
Buy ones designed for mountain walking. I have mountain hardwear ones, totally waterproof but really pricey. For riding I have the Rambo chaps and they are quite good. Get a cold bum though!!!!!
Toggi Xtreme over trousers are my choice they are brilliant really stop the wind and the rain, they are made like chaps with the outside zip so are perfect for riding in, you won't be disappointed!!
I've invested in some lovely Rockfish ones, they are fantastic, really comfortable to ride in and they look good too, with zips that go waist to floor for easy putting on/off.