NYC: Pelham Parkway and Riverdale; subway service is better from Pelham Parkway.
Westchester County: Orthodox communities in Mount Vernon, New Rochelle/Scarsdale (actually 3 separate communities), Mamaroneck (centered around the school complex there), White Plains, and Mount Kisco, all close to Metro North Commuter Railroad.
Passaic, Riverdale, Lower East Side, depending on your definition of yeshivish, washington heights. Queens (KGH and others) Brooklyn has a bunch of frum areas. Depending on how far you are willing to commute there is also Monsey.