For passenger, hands down the Niagara or the T1 Duplex.
Sounds to me that your actual favorite is the NYC C1a.
Actually, in all seriousness there was a fairly detailed proposal for the C1a. One of the 'regulars' on the YouTube NYC Group (Hugh T. Guillaume) has a twenty-odd page document describing it (unstreamlined, with what looks like a long Niagara boiler and a properly large firebox and chamber), and Al Staufer did an ink drawing of a side view at speed in NYC Later Power.
Aside from West Albany Hill, the Water Level Route was better suited to duplexes than was much of the PRR. As I recall the C1a was to have Baker gear to piston valves, so plenty of room for a conjugating shaft, and divided drive would have solved any major issue with main pins or buckling rods...
Of course, Arnold Haas to the contrary NYC had no particular need of 120mph power; even the successful passenger diesels were geared low, and the ICC order on train control that went into effect by 1951 put a damper on any speed increase that might have become more feasible as the Great Steel Fleet began to thin out in that decade. By the time PRR had worked the conceptual bugs out of the duplex, the entire market for new passenger steam had collapsed completely (1948).