WordPress.com! Of course, it has downsides (no plugins, limited themes, etc). But it has the added benefit of constant, automatic updates, reliable infrastructure and, well, it’s free.
If you plan on running a self-hosted WordPress blog, the biggest mistake you can make is to skimp on hosting, and I’d strongly advise against using a “free” host. Security, performance and uptime are all dramatically affected by your choice of host.