Drysuits are the very best option to keep you dry and warm when paddling and are ideal for cold weather trips on salt or fresh water.
For white water a drysuit with full latex seals is usually the best bet, although there are some available with double Neoprene gaskest on the neck. This means both neck and wrists and should keep out virtually all water.
Suits with neoprene or adjustable necks are more suitable for sea kayaking, touring and recreational use. Although this means the suit is not technically 'dry', latex can be quite uncomfortable and irritating so for extended periods of time on the water and neoprene is far nicer. Also the chances of having lots of water pressure on a neck seal when touring, even in the event of a swim, is unlikely. Sea Kayaking suits will also have a hood, which can make windy rainy day a lot more pleasant.
There are a number of entry methods, such as rear-back, leg, over the shoulder, and hinge sytems, all with pro's and con's:
Rear-back Entry - the original standard for drysuits, a well designed and fitted rear entry zip can be comfortable, and easy to check if it's closed. On the downside, if it doesn't fit correctly, the zip can restrict movement and feel uncomfortable. bulky zips can also mean its hard to pull on a vest style buoyancy aid. If you have a rare zip failure, then the suit is pretty much unusable. Many people need help to do the zip up.
Leg Entry - There's no restriction to paddling movement with leg zips, and these styles can also make it easy for ladies to use the toilet without having to undress and remove BA's. Any zip failure will leave you with a function cag section, so you should be able to complete your trip. These are also easy to do up yourself. Con's for this method - some can find the zip awkward between their legs, and the long 'body tube' that you have to pull over the head can feel claustrophobic. for those wearing higher boots, some can find the zip ends in an uncomfortable position.
Shoulder-Front Entry - This method is generally easy to put on and off yourself, and like a rear entry can be comfortable when in the right position for the wearer. However, depending on the zips used and the position on the paddler, they can still get in the way, restricting movement at the shoulder, or causing pressure at the hip-waist. As they come across the body under a BA, they can create pressure if you have the BA tight.
Hinge Entry - This system uses a zip that runs around the chest area, on the inner body tube under the outer spraydeck cover, leaving a couple of centimeters of fabric. When in the right position, these are comfortable, with no restriction of movement, either for paddling or swimming-walking, and it's usually easy for most people to zip themselves in. Zip failures leave you with with a functioning cag and trousers. Downsides can be the zip can press if the BA is tight, but the zip should be positioned out of the way.
These methods can be available with one or both of the two zip types - plastic or brass. Brass Zips are considered harder wearing, but is much stiffer than the plastic TiZips. Generally Leg Entry and Hinge Entry will use plastic zips as there is more bending and flexiblity required, where as Rear and Shoulder-Front Entry suits can use either, with brass found on the suits designed for harder wearing expedition or instructor suits.