Rambling, possibly-helpful non-answer: whatever you can get up and running with the fastest, and can use with minimum of friction. Blogspot, wordpress.com or posterous would probable all fit the bill.
Seems like about half of the most productive people/ most interesting blog posts I read are hosted at *.wordpress.com or someplace similarly humble.
But I mostly post this because I'm just finally finishing up a migration from self-hosted wordpress (which was working fine, but I wasn't happy with for various reasons) to octopress. This has taken me several months to get around to almost finishing (during which time I've gotten out of the blogging habit), and many hours of fooling with ruby environments, converting my old posts, etc., etc.
Hopefully the payoff for me will be simplicity, a better work-flow, new blog format that fits what I'd like to do better, etc. but it might have been a lot of wasted time and energy.