As with so many questions that get asked here at Lifehacker, there’s no single obvious and correct answer, and we expect that readers will have some additional comments to offer on what you might choose. All the services you mention will perform the same fundamental task: giving you an easy way of posting your thoughts to the world without requiring you to set up (or pay for) your own site. While there can be good reasons for doing that (and we’ve got plenty of guidance on how to do it, letting someone else handle the technical details lets you concentrate on what you actually want to say.
The difference is mainly a question of emphasis and features. Posterous is designed to be updated via email, which makes posting extremely simple. Tumblr emphasises brief, Twitter-style updates. Blogger offers integration with other Google account tools, and WordPress has an open and frequently updated code base which enables lots of good-looking themes. We looked at each of the options you discussed in our Hive Five roundup of blogging platforms last year, which will give you a more detailed overview, so we won’t repeat ourselves here.
One other point: this isn’t necessarily a choose-one-only approach. You might select Tumblr for quick thoughts which you update frequently, while using a WordPress blog for more in-depth but less regular musings.