Well, according to Microsoft actually :-)
The top 20 developer blogs that Microsoft adjudged to be the best are listed here.
The criteria for rating was:
|•||Frequency of updates - 20 %|
|•||Originality - 40 %|
|•||Writing Style and User Experience - 20%|
|•||Content and Technical Relevance - 20%|
Go ahead check out them blogs if you are interested in technology. You won't be disappointed.