For me the best by far was Paul Buchheit, because he was not only honest in a general sense (all the others seemed so as well), but also intellectually honest.

There's no one way to do things, what works for one won't work for another. His talk acknowledged that right off the bat, stayed true to that philosophy, and still managed to impart lots of wisdom (some implicitly)... while being very engaging. (Humor is underrated.)

Thanks Paul, and thanks to all the other speakers as well! This was my second startup school (my first was Fall 2005 in Cambridge - jeez, was it really that long ago??) and both times I've come away inspired and fired up.