Two categories(diesel w/streamlined cars & steam w/streamlined and or older Pullman style cars) with the option of three runner up passenger cars and one runner up locomotive for each category. Lets here it!
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I'll play!
Runner up first: PRR GG1 with three P70 coaches, a D78 diner and a heavyweight parlor observation on the end.
Diesel Electric: F40PH with Budd Heritage fleet baggage / dorm, coach, lounge, diner, and 10-6 sleeper. A good train for me at about 25 of my closest friends and a well paid staff to take care of the arrangements, food etc. 
Steam: CNJ Blue Comet Pacific #832 with three coaches, the diner and that classic parlor observation car.
I love those photos! I just by accident have a Harriman Ten- Wheeler pulling three Harriman passenger cars and a Pullman observation car on the outer loop.
My sound man Matt beat me to the punch. Nice shots Matt. Don
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How about a Reading T-1, #2124, and five SGL coaches? That would make my day!
The "J" pulling anything and a GGI on the front of any Fleet of Modernism cars.
The 611...nuff said. 
Interesting topic. As a matter of fact, I bought my first few O three-rail cars with the idea of making an excursion train.
If I were to do a model excursion train (as opposed to a full-sized one) set during the steam era or early-diesel period, I'd probably use several heavyweight coach-style cars (perhaps something like K-line's coaches). I'd also consider having a gondola car either at the end or mid-train with some sort of gang-way to allow the excursionists to go to and from the regular passenger equipment. I'd also consider a baggage car (perhaps lettered as a tool car) or one with open doors and railings to allow railfans to poke their heads and cameras out to capture the sights and sounds of big-time railroading.
I'm with Mister Lee. I know you are asking for coaches, but from my experience there is usually at least one coverted gondola with a continuous double sided bench seat running almost end to end down the center. Those would be the seats that get taken first, unless it's raining! I'm pretty sure no manufacturor in O makes them, so I've been pondering kit bashing to run behind my Shay and Potomac Eagle F7's. The Potomac Eagle used a passenger coach where they cut the window pannels to make a long rectangular open "slot" running the length of the car (both sides) They also removed the seats. This way folks could stand and move around and get fresh air. Of course, the PE only travels at about 15-20 mph.
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With what I have seen in local tourist train operations, I would pick a WilliamsBB 4-6-0, a mixture of light and heavyweight coaches, and baggage cars, still lettered
for their vareious original owners, and a couple of open flatcars with seats, roof, and waist high walls fabricated for them, run at the rear of the train, farthest from the
smoke and soot.
Get some NJ Transit coaches and slap them behind a C&O 614 Greenbriar.
