Depends on your needs.
Frontline Plus is good
Revolution also treats for earmites, deworms, & is a heartworm preventative.
Advantage is supposedly good.
I peronsally use Revolution, I can get it for $4 per cat(with 15 cats cost is important!)....and for the farm kitties it keeps the earmites away, & the heartworm preventative, IMO, is important for them in particular.