Trust me, even the experts don't know what the essential blogs are.
I could provide a more comprehensive list later, but a short "required reading" list would probably be these:
Just Another Fool
Seeking Truth (and probably The Mystic)
Dreams in Darkness
The Tutorial (while opinions of quality are torn, it's necessary for understanding many current blogs)
H(a)unting (while general opinion is that it sucks, again, it's had a lot of impact on the mythos)
White Elephants (at least up to the point that the blog was first wiped--you can retreive the archives by following the blog with Google Reader, though the comments have been lost).
A hint of serendipity
A Really Bad Joke.
Those are the blogs I'd say are most important to read first, because they've had the biggest impact.