Toughest neighbourhood? I think all Ballas or Vagos territories are the same.
About gangs in general, I remember first having 100% of the gang territory in my first playthrough. But still Ballas would always attack on of my territories in the south. After a long time I found out that I had to overtake some territory nearby again, because it was only light green on the map and there were still Ballas there on very rare occasions.
About the Varrios Los Aztecas, I found it always very weird, that they attack CJ even after the storyline missions, because CJ helped the Varrios Los Aztecas very much and it would only be logical if the became allies.
Fun fact, in the beta version you could fight gang wars in San Fierro and maybe also in Las Venturas. In the official strategy guide there is a screenshot where CJs stands near Zero's shop and the radar shows violet/dark blue gang territory probably of the San Fierro Rifa.