The best paid-for CMS that I have used (as a developer) is Sitecore.
It's a HUGE large-scale ASP.NET CMS and is relatively popular for .NET developers that work on huge websites. It's a bit of a beast, but it's a great CMS to develop on top of, and its feature set is fantastic when you want to build a massive website.
I forget how much a license costs, but it's definitely not cheap, around several thousand pounds. However, if you're dealing with a site that gets hundreds of thousands of hits per second and has several hundred thousand pages it's quite nice to use.