Guys - the biggest mistake that people make with banners is making crazy flashing animated .gif files.
First off, always note the specs: https://newschoolers.com/mediakit/?page=bannersspecs
Second - use elegance, and talk to the client about what the purpose of this banner is.
Ryan - you need to ask yourself, WHY does this banner exist? Who is it targeting? What measurable statistic are you trying to improve on your end?
If you're trying to get people to a sale, then the banner should have a clear focused message describing that sale. If the idea is branding and your goal is to have people recall your brand later when they're going to make a purchasing decision, you need something clean and very cool.
Same if you're promoting a contest, likes on your FB page... whatever. If you give everyone more of an idea of your goal, as well as who your campaign is targeting, then we can really get something effective together.
Also make sure to be really careful about targeting your buy. I assume you're going to use the self-serve system. It can be great, but I recommend doing a narrow geographic focus as well as a shorter but high-impact timeframe.
Finally, Banners lose their effectiveness about 1-2 weeks after posting. I highly recommend either using multiple different creatives all at once, or rotating them every 7-12 days. Bonus points if your marketing message flows across the entire set.