Three words: content, Content and CONTENT
This is the main reason I keep going to the same blogs for more. Nothing beats compelling content especially when it's well presented and offering valuable advice.
Some of the blogs I'd consider most excellent at the moment that are right at the front of my mind are:
Seth Godin's Blog:
http://sethgodin.typepad.com/
Legal Andrew:
http://www.legalandrew.com/
North x East:
http://northxeast.com/
Computer Zen:
http://www.hanselman.com/blog/
Andy Beal Unplugged:
http://www.andybeal.com/
And the main reason I keep going back for more i because of the quality, variety and richness of their content.