It really depends on what you're looking for, but usually you want a high revenue share percentage, a high CPA paid to you (you can find some amazing ones as an impetus to drive your sales), and a reasonably popular product that isn't in an overly saturated niche. In the interest of sleeping well at night, you might want to get a product that you can stand behind. My favorite example of a good affiliate program is https://p4p.partners/commissions/