Prior to the merger National's biggest base was probably Miami, although they had respectable operations (maybe 30-50 flights/day) in Houston, New Orleans and New York as well. None of these really qualified as a true "hub" in today's sense of the word, though.
National's 1980 route system resembled a backwards "L," with lots of east-west routes connecting Florida to California, Louisiana and Texas, and lots of north-south routes linking Florida to New York, Boston, Philadelphia, Washington, etc. They also served Norfolk, Newport News, Charleston and Savannah along the east coast.
Unfortunately, having their biggest base in Miami became a problem for National after Deregulation. Nobody in California was going to fly to New York via Miami, as it adds extra hours to the trip. The only point south of Miami that National served was San Juan, which was added just prior to the Pan Am purchase. Except for that one city, there weren't many places that National could funnel passengers to via Miami.
That's what made the expensive National purchase so problematic for Pan Am - it was supposed to give them the domestic network they had lusted after for years (but which the CAB continually denied them.) The addition of National's backwards "L" didn't do much to feed their transatlantic or transpacific routes. What they really needed were more lucrative transcon routes like JFK
, etc., and National couldn't provide those. The merger was also sort of pointless in hindsight, as Deregulation allowed PA to add domestic routes at will, so they dropped a lot of their own scarce cash on an airline that didn't give them what they needed. What's more, National - which could have been a post-Deregulation star if it had just more time to grow - was absorbed into the unwieldy, unstable beast that was Pan Am in 1980, and a lot of good people got screwed in the process.
I'm digressing... to get back to your point: National didn't really have any "hubs" per se, but MIA
was the biggest station, followed by JFK