I'll second the Concrete5 recommendation. It's MVC under the covers, a common sense architecture, highly customizable, an exceptional UI for content editors, and a good community. For content editors, we do a one hour boot camp and that is sufficient to bring anyone non-technical up to speed.
The CMS I use the most is Umbraco. It's an open-source ASP.NET CMS that I've used on a varied range of sites, from small to very large. It has its flaws, like most of these types of tools, but I recommend downloading it, and giving it a quick test.