Not cod liver, since it's quite high in vitamin A and most cod liver supplements are formulated for humans, not dogs, so I worry about getting the right dosage for my little guy. I do use salmon oil, which still has all the good fatty acids without being so heavy on the fat-soluble vitamins.
As has already been stated, cod liver has a lot of A which if your dog is eating a balanced diet, it does not need any extra. Fish oil from fatty fish like sardines or salmon is a great way to supplement omega 3s. I personally feed my dog canned mackerel twice a week instead.