Ask what you can offer the dog, rather than focussing only what a dog can offer you. The easiest dog to train is the dog that is right for you and your life.
Different breeds of dogs were bred for different functions. So different breeds have different instincts and different behaviour. The look of a dog is far less important than its breed behaviour. The breed instincts still remain in our pet dogs.

TAKE ADVICE
Breed societies and their websites are always convinced that their breed is the best. So many breed websites will give the good points and ignore the bad. If you get a good breed website which does run through the hereditary diseases, then you know the rest of the website will be reliable too.
Read Choosing the Right Dog for You by Gwen Bailey This book is now out of print but, if you can buy a second hand copy, it gives a complete list of inherited diseases in all breeds. There's also a reasonably independent website which might help - www.dogbreedinfo.com Another useful website, produced by Cambridge University, where you can search by breed, is http://www.vet.cam.ac.uk/idid/search.php Or read Breed Predispostions in Dogs and Cats by Alex Gough and Alison Thomas, Wiley-Blackwell, Second Edition 2010. This is a book which is important for anybody seriously researching this topic. Worth consulting to check up on a breed but not worth buying just for information on one breed only.
Here are some other things you need to think about.

HOW BREEDS DIFFER
Gun dogs will do things like retrieve, point game, swim after game and flush game on land. Their instincts are still active and they will be happiest if they can do some of this. Labradors and retrievers will retrieve socks and slippers, if there's nothing else - it is bred into them.
Dalmations, setters and springer spaniels are highly energetic and rather dizzy in their early years. Labradors too. Don't expect too much too soon but start kind and gentle training early.
Terriers were bred to shake rats and will shake squeaky toys instead! They were bred to go down holes and to be impervious to pain if bitten. So once they start fighting, they are likely to fight long and hard. They've got strong predatory instincts so will go for small pets and cats, unless they've been brought up with them. Even tiny Yorkies can be terriers at heart.
Guard dogs were bred to patrol areas, guard territory and see off the bad guys. They are likely to be highly territorial and will guard people and home. This instinct needs careful training. They will be loving and loyal to their family and their children, but may not be so accepting of the children's friends or yours.
Bull breeds were bred for their capacity to bite bulls and hang on! Nowadays their temperament towards humans is fine but they still have the hard bite. Rottweilers also have a big bite. Good training is essential.
Akitas, in particular, need experienced owners. They are not a beginner's dog and should never be bought as a first dog. Good training is vital.
Sheepdogs and herding breeds were bred to herd. They will often herd ducks, joggers, bikers, or even children, given the chance. Border collies still have all their herding instincts and need lots to do. Farm collies make poor household pets unless given lots of exercise and training. Border collies are so clever that they can be difficult to train.
Greyhounds and other chase dogs were bred to chase. If something - a cat or a hare or a small dog - flashes past them they will take off. They can't help it. It is their nature. Some need walking in a muzzle.
Hounds were bred to track, often in a pack. They may not care for toys and they are often emotionally independent. Dachshunds were also bred to hunt and go down holes. They are feisty little dogs a bit like terriers in temperament.
Cavaliers were bred to be pets. They are gentle beings and a good choice for first time dog owners.

Consider what you have to offer the dog, not just what the dog can offer you.
WILL THE CHILDREN BE SAFE?
Do you have young children? Very young children sometimes harass dogs, so be cautious. There's nothing wrong with big dogs like Rhodesian ridgebacks, Akitas and rottweilers but they have a big bite if things go wrong. Choose a gentle breed. You want a dog that is not over-sensitive, but patient and tolerant.
WILL IT BE A TOWN OR COUNTRY DOG?
Do you walk in the park with other dogs? Chasing breeds like greyhounds and lurchers may be set off by fleeing small animals including small dogs. You should walk them muzzled (nothing wrong with that, incidentally) - just to make sure. Untrained guarding breeds may be difficult around other dogs.
DO I HAVE THE ENERGY?
As a rough rule border collies need two hours exercise or a full hour with a chase toy to double the distance. Clever sheepdogs also need things for their mind. There are suggestions in my book, "One Hundred Ways to a Happy Dog." Jack Russells and many other terriers are also tireless.
WHAT ABOUT THE CAT?
Do you have a cat? Chasing breeds may be more likely to chase it and it only takes one snap to kill it. Never expect a cat to live with a dog that isn't used to cats (see how to choose a puppy). Terriers are fierce little dogs that may also have a go at a cat. Choose a gentle breed, like a Cavalier, so that the cat will be safe!
WHAT ABOUT VETS BILLS, AND OTHER COSTS?
Can you afford a large dog - they eat more and their vet bills are higher. All large breeds have a higher incidence of hip and joint problems. The giant breeds are worst for joint problems. They have a shorter life span - eight years is old for a great Dane.
Hereditary diseases in any breed make for big vet bills So check which breeds have hereditary diseases by reading "Choosing the Right Dog for You" by Gwen Bailey (www.gwenbailey.co.uk )
Miniature breeds may sometimes have problems like slipping knee caps. Dachshunds have back problems. White coated dogs are more likely to have skin allergies. Insure your dog for life with a good insurance like Pet Plan which covers all diseases. Cheaper insurance may not cover chronic conditions.
WHAT ABOUT HAIR AND TAILS?
Long haired dogs will require brushing every single day and will bring in more mud. You may need a car with air conditioning for very hairy dogs like Newfoundlands. They will suffer from the heat in a hot house so if you like high central heating, get a smooth coated breed. Poodles and bichon frisee will need clipping - either by you or by a dog groomer. All dogs will shed hair in the house so remember that black and dark hair is less visible!
Tails are still being cut off by breeders. But if you ask the breeder to leave the tail on, as the Prince of Wales did with his Jack Russells, they will usually do so. If not, go elsewhere. Dogs with tails find it easier to communicate with other dogs. Help dog lovers set a new fashion about this.