Obviously multiple factors, especially design, power and operative performance.
I would suggest three locos, two of which have been extensively modeled in O gauge and one in exquisite brass by Sunset Models in HO:
1. The Norfolk and Western Class J 4-8-4's - Roanoke built and maintained - beautifully streamlined, fast and powerful locomotives. Number 611 exists and may yet run again!
2. The New York Central J-1E Hudson and its subsequent streamlined brethren - all beautifully proportioned and fast passenger haulers. NONE exist.
3. The Nashville, Chattanooga and St. Louis Class J-3 570-580 series 4-8-4 "Dixie Class" semi-streamlined locomotives. These dual service locos were designed in Nashville, TN by Master Mechanic, Clarence Darden, and built by ALCO. Number 576 exists but likely will not be restored to operation.