I think the best rain coat is one that is knee length and made from a waterproof material, in which the water remains as droplets that slide off. After a few years that waterproof begins to degrade, but you can buy a spray that makes it work again.
To wear? I think that the best to wear would be a knee length one, as Bryony mentioned. A knee length one will keep more of your clothes dry, but it should be made out of a type of coated nylon. This material allows some air to pass through, making it breathable, but is also waterproof.
Definitely a water proof one, with a hood pulled tight and elastic cuffs to stop the rain getting in. I'd like it to be calf length, loose and as fashionable as possible.That would certainly be better thabn the three dollar emergency plastic poncho I keep in the car glove box.