There's no software that can be considered best. It boils down to what you can work with best and whether or not a CMS (should you go for a non-custom solution) is suited for the task.
If maintenance is your priority, then I'd go with the system that is not only well-coded but has a administration panel that you find intuitive. I use ExpressionEngine. It's my absolute favorite CMS. Not only is it flexible in terms of structuring and organizing your site, but it also has a nicely structured user interface. You also have the option to simplify the admin area's appearance by using a third-party admin theme. By default, you have three admin themes to choose from, so you're not stuck with using the one that comes pre-installed.
However, ExpressionEngine is a commercial application. Not only the application itself, but the majority of add-ons are commercial as well, so you should take that into account. I don't mind paying for quality software because I'm not a fan of having to mess with programming languages.
WordPress is good, of course, and the admin interface is very, very good. Coding and configuring your site can be a nightmare, however, unless you're well-versed in PHP. The out-of-the-box code is, well, somewhat cluttered. If you intend to have clean code, your developer will need quite a bit of time to trim the fat.
After ExpressionEngine, my most favorite CMS has to be Textpattern. It's not nearly as popular as WordPress, the community is small, but it's a darn good piece of sophisticated software, well designed, well structured, lighter than ExpressionEngine and not a commercial product, should that be of preference to you. The interface is very simple, very easy to use, a no-nonsense type of admin interface that may appear a bit spartan at first.
There are many more, e.g. there's MODx, also a open source CMS. I've used it very rarely, but it's been said to be good, so that's one you could check out as well.
By the way, before you decide, you can test-drive all these open source CMSes via demos hosted at: http://php.opensourcecms.com/. They have recent versions of Textpattern, MODx, WordPress, and many other applications.