The vertical is the offer's subject matter, and it decides more than the choice of source: different audience, different payouts, different speed and different ways to lose everything.
Gambling
High payouts, a short path to conversion, maximum complaints and the hardest account management. Banned in most ad networks, so it lives where there is no moderation. See gambling traffic.
Betting
The neighbouring vertical with a longer cycle: the person waits for an event. The audience knows the subject and has to be talked with, not pitched at. See betting traffic.
Dating
Mass audience and a low barrier: high registration conversion, small payout per registration. Works on volume, very geo-sensitive.
Nutra
Health and appearance products. Usually paid per confirmed order, which means a call centre stands between your work and your money. Approval rate matters more than the rate itself.
Finance
Loans, cards, brokers. High payouts, strict traffic quality requirements and many rejections on the advertiser's side. A vertical for people who already have a clean source.
Crypto
The audience seeks the topic out, so warm traffic is plentiful. The flip side is extreme sensitivity to reputation and to waves of interest.
Where to start
Not with the highest-paying one but with the audience you understand.