Susceptibility varies by region; your local shelters and vets may know what is working in your area.
Frontline Plus wasn't doing well in my part of Columbus, OH, so I was using Advantage II.
I also used Capstar to quickly knock out the adult fleas and reduce the laying of eggs. It is less expensive if you break the dog version into approximate equivelant doses (or a bit over) for cats. FDA documents showed it had a wide safety range (on my pc, not on my phone, or I'd post it.)