The answer to the question is "IT DEPENDS". The metric to measure a creative depends on what a creative is intended to do. The best way to find out is to establish a goal of the creative. Typically, the creatives can be bucketed to mirror the stages of the consumer funnel – Awareness, Research, Buy and Loyalty.
Based on each of these buckets, the correct metrics to measure the effectiveness of a creative can be defined. The creatives in the "Awareness" bucket should be measured on CTR, as the creative is indented to increase the awareness or catch the consumer's attention. The next level of creatives in the "Research" bucket should be measured on the engagement or interaction of the consumer with the banner or landing page or the website. Thus, the correct metric would be interaction rate, page depth, time spent etc. The next level of creative "Buy" section would be aimed at driving consumers to make a purchase. As we all would agree the correct metric to measure these set of creatives should be conversion – revenue, number of units sold, number of sign-ups etc. The final set of creatives – "Loyalty" set would be driving existing consumers to either continue the services they had enrolled in or cross-sell/up-sell for different products and services. Thus, measuring how the existing consumers are interaction with the website would be the best way to gauge the performance of these creatives.
It gets more and more complicated but the better thought through the measurement process is, the better results one would be able to measure.