The mayor for not enacting the city's evacuation plan
The Governor for being an incompetent, indecisive dolt. 
FEMA & the director of FEMA for being incompentent at establishing command and control and setting priorities
National guard commanders for not realizing this was going horribly wrong.  (note: so far, this hasn't been investigated, so it's possible that there was an awareness of hte problem)
The President & his staff for being so hands-off.

Now, it's important to understand that in disasters, the municipalities are primarily, and in most cases, soley responsible for evacuation planning.  The state government provides assistance in the form of national guard support and command and control capability, typically through a state police force.  FEMA typically acts as a bank to funnel money into agencies who are responding to disaster, but they do sometimes provide command and control guidance.  There is almost always a tense relationship between state governments and federal agencies in this kid of situation, because the federal help invokes a lot of additional bueracracy and loss of control for the state.  The president is ususally not directly involved, except for making sure that the right amount of attention is focused on a given issue.

Almost every major man-made disaster in history has not been the result of a single major fuck-up, but a series of small missteps that fit together in such away that a catastrophy results.  I think this is no different.  We had incompetence and lack of focus from the mayor all the way to the president, and that cost many people their lives on top of an indescribable amount of suffering by those that did survive.