MTH do a nice model of the triplex. I guess it comes down to what you mean by most powerful? High horsepower, tractive effort? The Allegheny chucked out 7,500hp at 40mph, so probably the highest most usable power. The boilers were good for 8,000. No modern diesel gets near that. A bit more info http://www.steamlocomotive.com/allegheny/
Either way, monsters, the lot of them! I can't wait till the Big Boy gets out on the mainline again.