I'm also going to go with Frontline.
For years I battled fleas, I bath the dogs and cats, bomb my house, spray the yard and a week later it was as if I didn't do a thing, my animals and house would be covered, it was a nightmare.
Then I finally broke down and paid the price for frontline. Got to the point that I figured with everything else I was doing I was already paying a small fortune so why not? In the end, it was the cheapest flea prevention I did.
The effect wasn't instant, but well worth the wait. But most important of all, it broke the flea cycle like nothing else. Considering I had a major infestation of fleas regardless of what I did, I now only see maybe two fleas a year and that's only if we have a hot humid summer. Considering I have two dogs and three house cats, that is nothing. Frontline even broke the flea cycle so much that my barn cats don't have fleas and they were never even treated. Just remember to treat all dogs and cats at the same time.
And best of all, frontline does not have an expiration date. I have actually gone through several entire flea seasons without a single flea now that the cycle is broken, but if I notice even one, I treat the crew and stop it in it's tracks, I only buy frontline every few years. And organic gardener me is so relieved at not having to spray the yard or having poisons sprayed in my house.