Which US airport historically has the most days in which VMC prevail?
According to NOAA here are the cities with >80% annual sunshine which I've used as a proxy for VMC.  You'll find these cities are all in the desert southwest region of the US where the climate is arid. 

SOURCE:  http://www.ncdc.noaa.gov/oa/climate/online/ccd/pctpos.txt
DATA THROUGH 2004

CITY, STATE            ANNUAL % AVG POSSIBLE SUNSHINE        NO. OF YEARS OF DATA
YUMA, AZ               90%                                   42
REDDING, CA            88%                                   10
LAS VEGAS, NV          85%                                   47
PHOENIX, AZ            85%                                   101
TUCSON, AZ             85%                                   53
EL PASO, TX            84%                                   54


To investigate long-term trends you can access historical data at the NCDC website and analyze the data for specific weather reporting stations.