Actually Wordpres
is a content management system. It manages your content (blog posts, static pages) doesn't it? (Answer: Yes) It just happens to be geared towards being used for weblogs rather than 'regular' websites.
The best CMS depends on exactly what you're after from your CMS.
There are commercial, enterprise systems costing hundreds of thousands of dollars used for example on the mega-websites out there and things like news websites (think CNN). For example,
Vignette.
There are many, many open-source systems for eager website owners or folks on a budget. For example, those listed on
Open Source CMS.
Possibly, the best CMS for any particular website will always be one built from scratch with an infinite budget, infinite developers and an infinite development timescale. Unfortunately none of those are available in reality, hence the reason for so many ready made (and excellent) CM systems exist already. You just have to look at them and choose the one which suits you best.
Finally, alot of people swear by using Mambo for their website. Personally after using it for around 6 months on one of my sites, it wasn't for me. But if the shoe fits...
Cheers,
Salathe