depends on what you're trying to tackle.

fleas(4 wks), ticks(2 wks) - Frontline Combo would be the one you're after fleas in house and on cat should be gone by the end of the month. depending on how bad the infestation.

fleas, mites, and some worms - Stronghold/Advocate would be a better choice (great for indoor or outdoor cats)

as these cover the full life cycle you only have to treat the cat once a month and no treatment for the house needed in most cases. these kill the fleas and do not repell. you would need something like Indorex or RIP fleas to spray the house if you still have an issue in the house.

these you can pick up from your vets. Just give them a call ahead to be sure they have them in stock and they will put some aside for you.

Also if you have fleas you most likely have worms as ingested fleas lead to worm infestation. So speak to your vets about worming treatment. Drontal, Milbemax are pill treatments that you can use. Panacur is a paste and Profender is similar to the flea treatments as you can place it on the back of the neck. but be sure to not place it on the neck within 3 days of placing the flea treatment on. Sometimes they can irritate the skin. These need to be repeated every three months. Be aware that any of these can irritate the skin and keep an eye on the application area and see your vet if irritation occurs.

hope this helps.