the most important is relevance - it has to work with the content and where the consumer is in their purchasing decision (are they researching, ready to buy or are they a little shit without a credit card in which case only CPA will work). payout and conversion rates tend to work like a weighing scale. If one is high, most likely the other is low. Ideally you are looking for an offer that is an exception to that rule... or as close to it as possible.