When you are scuba diving, there is no such thing as a perfect wetsuit, but modern manufacturers have come close to perfection. The area you will dive in will determine whether you need a dry suit, a wetsuit, or a semi-dry suit. While dry suits are meant for cold dives, wetsuits keep your body warm in waters of moderate temperatures.
If you are going to scuba dive in a warmer area, you have the option of buying either a full wetsuit or a spring suit. A full wetsuit has full arms and legs while a spring suit has shortened arms and/or legs. Spring suits come in many combinations - a torso-only design, bicep length arm sleeves, or full length arms, but mid-thigh length pants.
A wetsuit called steamer cover your entire body except the head, hands and feet. Some steamers have removable arms or legs and are called convertibles. A shorty covers the torso but has short legs and arms.
Regardless of design, there is a standard way to measure a wetsuit - using the thickness of the neoprene material. A suit that has a thickness of 3mm (1/10th inch) thick in the torso, and a thickness of 2mm in the arms and legs is designated 3/2. Use a 3/2 one for dives in water that is 60F (15.5C) or above. If you need more warmth, you will need a 4/3 which is thicker. Some suits have separate numbers for the torso, arms and legs.
Like any other sports clothing, your wetsuit must fit well. Flexibility is important to allow easy movement underwater. The suit should not chafe or bind. Since you will be using your arms to navigate underwater, it is important that your suit allows for a free range of shoulder movements. It should be snug but not squeeze. Restricted blood flow near the skin surface will cause soreness and fatigue and shorten your dive time.
Secure suits will have fewer number of sturdy seams - wither glued, welded or uni-body. The material is most likely to split at the seams. Bad seams can also cause sore spots and adversely affect flexibility. While suits which are seamless or have fewer seams are best, some suit designs have to have seams because of their extra features like hydro-lock collars, integrated knife holders, removable headgear, and fire-skin linings for wicking.
If you have to dive in colder temperatures, you must have a good dry suit to keep you warm. An adult can die in a matter of minutes if exposed to waters of 50F (10C) or less. Dry suits come with attached booties that seal off your feet. They also have watertight seals around the neck and wrists. You must wear thermal underwear with your dry suit. The dry suit has sealing that keeps it dry, while the underwear has wicking that keeps moisture off the skin. In case of wetsuits, the design allows a layer of moisture which is warmed by skin.
Try on lots of models before you buy so that you find a suit that fits snug and yet allows maximum flexibility. You will be using your wetsuit for years, so invest the time in finding the right one.