Which are the best American universities/colleges for observational astronomy? I'm a French student and I'm planning to go to the United States for a graduate program in astronomy.

When I will arrive in the U.S. I will begin by the first year of graduate. Then I will pursue with a Ph.D. and some postdoctoral research.

I would like to know which are the best American universities/colleges for what I want to do.

Concerning what division of astronomy I am interested in:
I'm not interested in the conception of telescopes, instruments and other systems. I'm not interested in engineering. And I'm not interested in extremely theoretical astronomy either.
I'm interested in observational astronomy.

Regarding what subfields of astronomy I am interested in:
I'm not interested in extragalactic astronomy or cosmology.
I'm interested in planetary astronomy, exoplanetology, exobiology, SETI, stellar astronomy in the solar neighborhood.
I'm not interested in things that are less than 2 Astronomical Units or more than 70 light years away from us.
Out of curiosity: where does this limit of 70 lightyears come from? Exoplanets can be detected to way beyond that. This strikes me as a strangely restrictive limit.

Be that as it may. There are several great astronomy departments in the US, and it really depends on what you want to do exactly. For exoplanets, Harvard would be a great choice. Professor David Charbonneau is a heavy weight in the field.

Similarly, many of the other renowned Universities have good astronomy departments. Yale, for instance, or Berkeley (although since the Geoff Marcy incident I imagine the exoplanet department has taken a hit). 

So at this point it is not really a matter of choosing 'the best' University. I suggest you have a look at some papers by the professors at different Universities, to get a sense of what you'd like to work on. 

And don't forget: for your research to be fruitful the environment is also very relevant. Just have a look and see what city you'd like to live in.