This depends on your campaign goals. If your goal is to increase awareness of your business, specific products, or events, then you want viewers to see your ad multiple times. This means you should use strategies that maximize impressions. If you want people to interact with your ad and visit your website, then you should use strategies that increase your clickthrough rate, or the percentage of people who click on your ad when they see it.
See the section on campaign goals for more information about how your campaign goals determine your strategies for types of ads, targeting, and how you evaluate your campaign.