There is probably more hot air expanded in the outdoor press on the subject of waterproof shells than just about any other subject. Without going into all the contradictory figures that every manufacturer produces to prove their waterproofs are the best, we can safely state that you will get wet when wearing waterproofs, but not, in certain circumstances, as wet as you would get without them!
Waterproofs are also known as Hard Shell. The best waterproofs are not only waterproof, they also "breath" (ie let your sweat out) to some extent. Anyone who has had the misfortune to use a non-breathable waterproof will realise how effective modern breathable waterproofs are, but unfortunately even the best, as they rely on a gradient of higher humidity on the inside passing to lower humidity on the outside, do not tend to work as well when wet (which is of course when you need them to work most).
Apart from rain trickling in the hood and sleeves, the major source of moisture is your own sweat. The ability to disperse this sweat is known as "breathability". In general, the more expensive waterproofs tend to be the most breathable - however even these have their limit and that limit is currently well short of what a sweaty body carrying a large rucksack up a steep hill can produce.
The result of all this is that they are not as breathable as they really need to be and you will be better off trying to avoid wearing them as much as possible.
Although the most expensive waterproofs tend to be made of the most breathable fabrics, this does not mean that cheaper waterproofs are no good - it all depends on the use you intend to put them to. For instance it makes little sense to carry a top end waterproof in the alps where it will just be extra weight in your rucksack as it will almost certainly weigh more than a cheaper one. Take the cheap lightweight one - if the weather turns so bad that you actually need to wear it you'll probably be descending anyway.
We also certainly cannot tell you what the "best" waterproof shell is; this is because that too depends on what you want it for, but there is a good selection here, of some of the best designs available in the UK. Most of them have been selected for their ability to take a large Ordnance Survey map in the chest pocket (something many foreign designs can't do) and for the ability of the hood to cover your helmeted head leaving just a slit for you to peer out through at the Cairngorm blizzard - amazing how many "top end" jackets can't do this properly.
Heavy Duty Waterproofs: Tough but still surprisingly lightweight - designed for climbing and walking in winter conditions.
Lightweight Waterproofs: Lightweight for situations where weight saving is more important than performance or durability.