What cities in the U.S. have the best and biggest financial sector?

NYC has the biggest.  And it's not even close.

 

Philly, I think, would be next in line on the East Coast.

 

San Fransico ans Seattle I believe rule the West Coast in that field.

 

Charlotte and Atlanta in the Southeast. 

 

I would imagine one of the Texas cities has a decent sectors.

 

Chicago of course in the Midwest.  Cleveland and Cincy are nothing to thumb your nose at.

^I'd guess Boston is ahead of Philly b/c of the mutual funds and venture capital, but this is obviously a vague question.

Having lost National City, I'm sure Cleveland fell significantly. It still has Key and, of course, the Fed.

Ernst & Young has a significant presence in Cleveland.  Other firms are downtown too, such as Smith Barney but maybe on not such a large scale.  Morgan Stanley employs quite a bit.