Bug Off (granules, not chewables) from Springtime. Was very very hesitant at first (they hardly got even half of the recommended dosage the first few weeks), but it works, no adverse reactions, and vet approves. . . they still don't get the full dose (just half).
ETA: I know it must be doing something because my dogs go through heavily wooded areas where people have warned me about ticks, have been to shows where people have complained their dogs came home with fleas, etc. and I have not seen one of either on my crew since starting the supplement.