I would like to get a list of the most  common disease/conditions in the U.S
I also would like to get the ranking of each disease.

This might be of interest to you:

http://answers.google.com/answers/threadview?id=31114

Thanks for the link.

I would like to get the same answer but 10 diseases are not enough...
I would like to get a list of 25-50.

There are not published data for the most common medical conditions in
the US, in part because there are insufficient data from the billions
of outpatient medical visits that occur each year, and data collected
from visits to emergency rooms and hospitals by condition is biased.

The CDC has excellent annual (i.e. longitudinal) data on the most
common causes of death via www.cdc.gov/nchs that extends beyond the
top 10.

I hope this helps.