Dog intelligence: ranking & sources

Dog intelligence is the ability of a dog to learn, think, and solve problems.
A white Standard Poodle in the Scandinavian clip, tail not docked.
Dog trainers, owners, and researchers have as much difficulty agreeing on a method for testing canine intelligence, as they do for human intelligence.
Certain breeds, like Border Collies and Golden Retrievers, are generally easier to train than
others.
It is worth noting that these descriptions are relative to other dogs, not relative to the world at large.
The ability to learn and obey commands is not the only possible measurement of intelligence.
Dogs are pack animals. "Pack" means that they understand social structure and obligations and are capable of learning how to behave around other members of the pack.
Adult canines train their young by correcting them when they behave in an unacceptable manner (biting too hard, eating out of turn, and so on) and reward them for acceptable behavior (by playing with them, feeding them, cleaning them, and so on).
Although you wouldn't want one to balance your checkbook, dogs can count. They can also understand more than 150 words, and intentionally deceive other dogs and people to get treats, according to a psychologist and leading canine researcher.

Ranking of Dogs for Obedience/Working

Border collie. According to several behavioral measures, research suggests that dogs' mental abilities are close to a human child age 2 to 2.5 years.
Credit: iStockphoto/Iztok Noc
Intelligence by Breed
Based on a dog trainer's survey.

Brightest Dogs
Understanding of New Commands: Less than 5 repetitions.
The Intelligence of Dogs is a book on dog intelligence by Stanley Coren, a professor of psychology at the University of British Columbia in Vancouver. Published in 1994, the book explains Coren's theories about the differences in intelligence between different breeds of dogs. Coren published a second edition in 2006.
Moreover, the Dognition site, based in North Carolina, uses questionnaires and games to help dog owners determine how empathetic their pet is, how clever it is at solving problems, whether it is choosing to ignore you or genuinely doesn't know what you're saying, and more. 
Sources: Wikipedia article "Dog intelligence", petrix.comsciencedaily.com