Frontline Plus is what we use, but it is expensive (about ~$12/dose for a dog 45-88 lbs)
BioSpot is highly recommeneded by Drs. Foster and Smith and is is much cheaper (more like $3/dose). I don't know anything about it, other then it should not be used around cats and it is not water resisitant. Fosters and Smith catalog really favors it, my mom's vet doesn't care for it...( Ithink that is the one she told me) Here is more info on it BioSpot