Pittsburgh Is a City, Not a Region—It Has No "Largest City" Within It

Pittsburgh is itself a city—the second-largest in Pennsylvania (pop. ~303,000, 2020 Census), after Philadelphia.

It is the seat of Allegheny County and the core of the Pittsburgh Metro Area (~2.3M). The question likely confuses the city with the metro region or assumes Pittsburgh contains other municipalities (it doesn’t; it’s a consolidated city-county for some functions, but surrounding towns like Mt. Lebanon, Bethel Park, Monroeville are separate boroughs/townships).

If you meant largest city in the metro area, it’s Pittsburgh. If you meant largest suburb, it’s likely Mt. Lebanon (~32,000) or Bethel Park (~33,000).