My cheap (20-25GBP) Polaris waterproof over-trousers do a good job: the taped seams keep the rain water out and the overall functionality is great (elastic belt, zipped ankle/leg, reflector band etc.).
My main 2 problems are:
1) I tend to put things in my trouser pockets, but with the over-trouser they become inaccessible !
(yes, my fault really)
2) They are not breathable so they make me sweat...I suppose you get what you pay for.
I hear that the eVent jacket is top-notch as a waterproof layer on top of a merino base layer (which keeps you warm, even when damped due to perspiration or water ingress). If you have the money, that should be at the top of your list.
eVent fabrics: our technology
Personally I still use a basic waterproof jacket (K-Way type with zipped front)...it's low tech so it works only when there's no strong wind (it is the wind that pushes rain drops into even the most hidden "holes" in the jacket design)
My cap is made of Gore-Tex Paclite, which I don't find very breathable so it gets damp inside...because of head sweat !!
Looking forward to hearing more on that subject !