Our favourite WCMS is Umbraco. It gives us complete flexibility to build whatever our clients require. Fast. We never end up wasting hours trying to shoehorn requirements into a solution that does not quite fit the bill. The support is great and we have had good experiences with our clients picking up the user interface very quickly. There are nearly 90,000 sites using Umbraco around the world - from small retailers to global companies including Vogue, Heinz, SanDisk and Microsoft.
Our second WCMS of choice is WordPress. We use this either because our client is used to using it and has data already set up or when specific plug-ins are required. Over 73 million sites use WordPress. This number includes a lot of small blog-sites but it is also used by UPS, Dow Jones, Time, CNN, CBS and many other huge organisations.