Although I may be biased due to my experience with Tumblr, I find it to be the easiest blogging platform to use. The dashboard is easily found with specific buttons that enable the blogger to post text, videos, links, music, etc. Posterous is in the middle in regards to ease of use. WordPress is by far the most complicated; however, this is not completely a negative aspect. This complication is a result of its numerous features. It will take time and dedication to truly understand and master WordPress.
In an attempt to decide on blogging platform, I have established blogs on Tumblr, WordPress, and Posterous Space. As I am already familiar with Tumblr, having my own personal blog for a couple years now, I will mainly be focusing my attentions on WordPress and Posterous.