I've found through experience that Ivermectin is the most efficacious (IMHO) when dealing with ticks, mites, fleas and other parasites on dogs. I administer Ivermectin to my 2 dogs in the form of powder bought from a local pharmacy, every 2 weeks, dissolved in a little milk. As already stated, it doesn't stop ticks getting on the dog, but once they bite the dog, it's game over for the tick. It keeps my dogs are relatively tick-free.
Edit:
Forgot to mention: It's not suitable for certain dogs, such as collies, old English and Shetland sheepdogs.