I'd contend, if you are trying to measure great creative experiences by one campaign or one email, you are wasting your efforts. That's why I so often discount awards given to campaigns for great creative; it's such a contextual evaluation that has little to no merit in my book. Creative is a part of the success overall, a visual representation of the work and in the email world, in my opinion, only 30% of the value. The other 70% should be attributed to targeting, timing, context and promotional strategies.
Not to discount good creative, but it's really difficult to assess great creative as an iterative process, it's so subjective. Here are several ways to assess performance of creative and get some idea of whether you're hitting the mark.
Click Map overlays will show you click density statistics of how your audience clicked on certain images or links within your email. That's a start to understand if you accomplished your mission in a direct response effort or if you are trying to understand the ranking and interest of your audience by link association, but hardly a complete measure of good design.
We have Heat Maps that allow you to test discrete focus groups on their eye movements within the email in different email environments. This will allow you to see how the eye scans, tracks, and paths within. While this has proven insightful, it doesn't tell you if you have created great creative experiences or even if it's good design. It only measures the tasks and how the consumer scans the email.
Creative competitive analysis is a critical baseline to understanding how your experience stacks up next to your competitors. We typically line up like emails from competitors and look closely at tone, design, modularity, promotional tone, use of images, use of navigation, linking strategies, viral strategies, and the general communication design. This baseline helps all team members form their own opinions on where you are on this scale.
Focus groups are critical to assessing creative -- but unfortunately, not many in the email space have access to these groups outside of internal audiences. Receiver reaction is some of the most valuable data you'll get -- qualitative, unbiased and able to balance all your other analysis. This typically comes in the form of offline intercept surveys, but you can enable this for email as well on direct response and custom landing pages, soliciting this type of feedback in the form of exit surveys. I've also seen some organizations use email focus groups and send out the campaign pre-campaign to get feelers on the best messaging strategies. A lot to administer, I know, but in the absence of good feedback most are just guessing.
Remember, great creative is about the experience, not the message; the reaction, not the design -- and it's about building on each experience, not a single email.
When creating a logo for your business, I used to advocate taking out every business card you've ever collected and placing them on a table. You'll be amazed at what pops, and if you categorize them you'll see a trend in aggregate -- what is good and what's missing; by looking at them en masse, you see an amazing level of detail. I recommend you do this with your email from time to time for a real-world view of the entire experience your email channel delivers. That's the true judge of great creative.