What is the best CMS for websites that allows flexibility for both the developers and contributors?

I think that, instead of thinking about traditional CMS software, you need to take a serious look at enterprise-focused CMS software, which combine document management, portal/intranet, enterprise social networking, team collaboration, integration with other software systems and many other features. I would recommend checking commercial open source offerings, such as http://www.alfresco.com/ or http://www.exoplatform.com/.

Definitely have a look at ImpressPages (http://www.impresspages.org). Content editing is extremely simple and straightforward - inline editing and drag&drop. Content is separated from design. And behind the curtains you have a lightweight framework you can adapt to your unique needs. Have fun!

We use eZ Publish specifically because of that need for flexibility, and also because it has both a vendor (eZ Systems with a very fine engineering team) and a community behind it. That said, think about what it is that you want your content editors to be able to control, and what you don't. Traditional wisdom is that design decisions should be decided for the whole organization and then implemented with code so they apply automatically, making the site consistently branded and beautiful. Perhaps editors are given some flexibility, but this is circumscribed so they won't have enough flexibility to wreck the design. When implemented properly, this works very well, but sometimes can cause frustration to those editors who are also designers and want to create their own layouts on the fly. But the tradeoff for consistency is generally worth it, and the new layouts can be added easily with a bit of code. I hope that helps.