I usually use either Trifexis or Heartguard and Comfortis, depending. It's cheaper to buy Trifexis because it takes care of internal parasites, fleas and ticks. If you buy a year supply at the office I work at, you get 10% off. Since I work there I also have a 50% employee discount.
Mom To~ Dakoda (b/t GSD), Simi (b/w DSH), Nashoba (sable GSD)