All of the evergreen niches. Niches that deal with high volume in the business world, as for example finance, loans, credits, pharma - you name it. Value of a new client in these businesses is significantly higher compared to smaller niches, therefore you'll likely experience higher payouts.
Despite higher payouts, all of those niches offer a huge possibility for longtail traffic that is not too difficult to rank for while highly converting.
As you mentioned coupon websites, try to look for something that keeps your clients returning. For example supplements can do a pretty good job for returning customers that come back and keep buying.