I have experience with MT, WordPress, Serendipity and Expression Engine. My work in IT at a university has given me an opportunity to explore blog software somewhat extensively.
Since version 2.0+ of WordPress, I`d have to give it the nod. As has been pointed out, there are hundreds of free templates, all of which can be (relatively) easily modified. There is now a broad range of some very good plug-ins. It`s quite stable. Installation and maintenance is simple. And, the WordPress developer community is alive and strong.
Here is a dated, but still helpful overview of blog software offerings: Blogs
(If anyone finds an updated version of this chart, please post it here.)
A more recent review and chart from the folks at USC: Blogs reviews Blog Chart
And a quick review from a great blog resource site: Blogs
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