I had a feeling Blogger would be number one for reliability, due to its Google ownership. Wordpress isn't far behind, so that is a great thing.
Tumblr really has a bad uptime record, 47.5 hours downtime? Come on, no one want's that!
Numerous people use free blogging services.
Today, came across this site that tested the blogging services.
Royal Pingdom » The most reliable (and unreliable) blogging services on the Web
Wordpress is best blogging service as far as i know.
blogger is the second choice on this
I never tried any other blogging service so i have no idea of telling the unreliable service
Wordpress and Blogger are the most reliable services for blogging. But if you're asking for the most trusted blogging sites that are the .edu and .gov!