Wordpress on your own web hosting. Seriously. I've used Blogger, Posterous, Tumblr and Wordress.com and pretty soon I find myself in a situation where I either need to pay for some basic feature (Wordpress.com) or not have access to that feature at all (Tumblr, Posterous). With my own Wordpress installation, I can install any plugin or theme I want and since I already have Webfaction as my hosting it doesn't cost any extra.