There are a million and one CMS' that do a good job but the interface and usability of it let the entire system down (like a lot of websites out there).
Whenever I need to develop a bespoke system for content management I always try and draw on my past experiences and those of my clients to work out what works well and what doesn't. So each time I do one there is a similarity to the last but with some extra tweaking to make it that much better.
So the question is what CMS interface / features have you found a pleasure to work with and why?
Note: This could be editing pages, products, sitemaps, just about anything you needed to manage through a CMS