It depends on lots of variables.
The answer is in part regional (how bad your bug problem is, whether there's resistance already to older products: some products just don't seem to work well any more in some places), in part lifestyle (how often your dog gets in water), in part genetic (some dogs are sensitive and don't tolerate some products well), in part owner compliance (some owners can't manage to properly apply topicals and get it all over the fur where it's useless, or forget to reapply in a month), and in part based on what you need the product to do. There are so many variables.
If you're at a loss, I would start with your vet, where you can go over things like: Are the fleas in your area suspected of being resistant to the old OTC products? Does your area have ticks/tick-diseases? Heartworms? Do you need protection against intestinal worms? Or are you covering HW and GI worms with a different product that's separate from your flea prevention?
I personally use Advantage Multi (called "Advocate" outside the U.S.) because it's an economical all-in-one that protects against fleas in my area very well, defends against heartworms, lungworms, and GI worms, for less than US$10/month (RX only). However, it has to be applied correctly, and the dogs stink for a day or so, they have to be kept out of water for a few days, and you shouldn't pet the area where it was applied while it's fresh. Those are drawbacks. Some people like the convenience of pills to avoid those things. Some people like to separate HW from flea prevention so that they can stop the flea prevention in the heart of winter when there are no bugs. There are a thousand permutations of variables that can support one choice vs. another, but your vet can help ask the right questions to get good prevention for all the bad parasites in your area.
If you're just looking for something natural as a repellent, rubbing the dog daily with products containing diluted neem oil, cedar oil, oil of lemon eucalyptus all may help. Wondercide makes a convenient spray repellent for dogs with cedar oil.
If you have a bad bug problem, you may also need to have a plan to keep them controlled in your yard by spraying (in addition to protecting the dog) -- you can by hose sprayers that are pre-filled with flea-fighting yard insecticide at home stores, or Wondercide makes a natural one.
Last edited by Magwart; 08-23-2017 at 09:25 AM.