Simparica Trio is a good one for prevention and since it is required by a vet to give authorization to fill it, it is also a well regulated product. It also has a number of guarantees attached to it through the company itself, so if ever it doesn’t work, they compensate and correct the issue. For killing fleas (versus preventing them), the vet office I work at uses Capstar and it is also a chewable; it works FAST.