The best backpack rain cover will keep your backpack as well as your head and neck dry. If the space between your head and neck and your pack is not covered the back of your pack that is against your back will get wet. The rain will flow down your back onto your pack. The padding on the harness will absorb the water increasing your pack weight and discomfort.
I have a backpack fly I got from Eastern Mountain Sports. It had an attached hood that covered your head and protected the space between your pack and neck. This helped to keep the harness side of my pack dry.Unfortunately I can only find a similar set up by Outdoor Research.
Your backpack fly should be an addition or a continuance of your personal rain gear complete with it's own hood for your head. While your personal rain gear should have a hood for when you are not wearing your pack, your pack fly should have a means to protect the underside of your pack.
The backpack rain cover should wrap around your pack. It does not have to wrap completely because if it did you would have to undo the straps on the pack harness to feed the straps through some openings. The harness straps would prevent the rain cover from completely surrounding the pack.
The material the rain cover or fly should be waterproof not just water repellent. Much like the fly on your tent, it should have a rubberized coating. The coating can be on the inside to protect the coating from abrasion and tears from brush and branches. More expensive rain covers have a rubber coating between two layers of polyester or nylon to protect the waterproof membrane on both sides because the cover can rub against your pack buckles and straps.
The backpack cover should also have adjustable draw cords to tighten the cover to your pack because not all packs are the same size and have the same shape. Elastic draw cords ( dynamic ) are not as strong as static or non stretching draw cords and can be pulled off your pack if caught on a branch. Static or non stretching draw cords are stronger and the spring loaded adjusting clips act as a safety to prevent tears.
The pack fly should be able to remain on the pack should you decide to stop for a coffee break or just to relax and enjoy the views. Even though you won't be able to see too far in a downpour. The cover should also allow you to access your pack without completely removing the cover.