We build all of our websites on the WordPress, self-hosted platform (not WordPress.com). There are myriad reasons for this decision, but the main reasons are:
WordPress is easy for the client to administer and you own 100% of the content
WordPress is open source software, so it is free and updated regularly
WordPress powers over 30% of all websites in the word, including HuffingtonPost, WashingtonPost, etc.
It allows you near infinite customization capabilities
WordPress is great for search engine optimization