What is the best dog vitamin to use?

Dogs that are fed a high quality diet do NOT need vitamins. However, if you're making your dog's food, I sure hope you've done all the necessary research to make sure this dog's getting everything he needs. If you had, you'd know exactly what vitamins to add based on what you're putting into the dog's food. We don't know what you're doing, so we don't know what necessary things are missing in your dog's diet. A lot of research is a necessity if you take on responsibility for your dog's nutrition.

Your dog shouldn't need any vitimins or supplimentsunless your vet has determined there's something missing from the dogs diet. Some dogs have troubles recessing foods with grains or certain protiiens so the cure for that is usually a change in food. A good sound commercial food has all the vitimins and nutrients your dog should need.

If you have a dog that doesn't eat well or needs a specified diet as prescribed by your vet then you should ask the vet if you need to be supplementing the dogs diet with anything else. Giving too many suppliments, too much oils etc can affect your dogs healthier in similar ways to dogs that are not getting enough. For instance many people have heard adding oil to the dogs food makes it more digestible for the dog as well as improves the dogs coat. This statement is true, home cooked food is usually a tad lean and doesn't always have all the nutrients needed for a dog and one with dry skin, dull brittle coat can do better with the added oil, however a dogs on a good diet that has all the nutrients in a balanced diet will end up with a greasy coat, patchy skin and loose stools if given the same amount of oil that the dog lacking in it needed.

Old saying, if it isn't broken.... Don't fix it.

I've never used vitamins except for the bird. She eats such junk. My husband feeds the blue chow chows kelp tablets because he thinks it makes them more 'blue'. He tells new owners of blue puppies that too, but there have been many champion blue puppies over the years, none of which, to my knowledge, kept getting kelp. A properly fed dog doesn't need vitamins. I take a multi because my eating habits are HORRID. (I'm a pilot and I eat whatever, wherever, when I remember or get hungry)
EDIT; cancer is in the lines of SOME breeds of dogs, not all, and there has been no link established that shows any supplement to prevent cancer in dogs.

Unless your vet specifically prescribes extra vitamins, don't bother. All good dog food contains all the vitamins the average already healthy dog needs. It's as easy to over-supplement as to under-supplement and you may in fact do more harm than good by giving more.

Yes, there is a lot of cancer around in dogs these days, but fact is what will be, will be. Some cancers run in bloodlines in any case and in others, it's more important to consider things like spaying in the b itch. Not so much male-related cancers in dogs however.

If you are feeding a home-made diet, then it is perhaps more important to make sure you use the supplements needed however. And for that, you need to base what extras you give, on what diet you are providing.

vitamins don't prevent cancer.

I am sure there are websites which try to sell you sea-front houses in Arizona. Which means there are tons of websites with misinformation. Particularly if a website tries to sell you something. Take something like the mercola website. While not everything is crap on there, there is a generous amount of crap on there and at the bottom of the website they try to sell you that crap. Which means it is an absolute useless website to get information from because you can't trust them and because they have financial interests in making you buy their crap.

Give your dog a good quality kibble and that will be all your dog really needs. However you can use other food for enrichment. While raw bones still do have some dangers, they do provide a lot of stimulation for a dog. I personally think it's worth the risk, so not everybody might think so.

If you want to boost the immune system, exercising and mentally stimulating the dog is going to be more important.