WordPress could be the most popular script around, due to it's ability to create both a blog and a CMS. It has virtually no weak spots, because it's developers always manage to come through for it and making bug fixes in nearly less than a week. It also manages to combine plugins and themes to make almost any WP installation different from the rest. The best thing about this package is IT'S ALL FREE!!
It's only downtime I've seen is a breach in WP 3.3.0's multisite problems, in which I had to contact my host for. About a week later, they improved WP so that it wouldn't have that breach anymore. As always, the developer team of Matt Mullenwag has come through for the community.
The biggest thing is design. Users can design pages the way they want. Users can use free themes from the themes directory. And, users get all free plugins, plus pages, categories, slugs, permalinks, ability to turn a blog into a full-blown CMS, and ability to have a blog and a front page at the same time. The power comes inside the WordPress package available for download at wordpress.org.
I love WP! I'm the admin on this site, and it comes with everything I need for a CMS and a BLOG! Best software ever, and I'm sure it's users agree with me.