Well, just my 2 cents but based on trying to put together the best SP cars I could actually find.
And the winner is - Lionel's 2011 Shasta Daylight set and two car add on.
OK, they are 18" cars and not scale accurate to anything like the extent that the GGD cars are. Also, they and Lionel's earlier Daylight offerings are not easy to find. But they are aluminum with a fine painted finish and have nice interiors. The set is also accurate in that all the cars are smooth sided save for the fluted-side observation car, which is what the prototype had.
The windows, which do protrude from their apertures, are not nearly as troublesome as Jack's picture above seems to illustrate. But again, I am not a purist about these things.
If I was a purist I'd regard the MTH SP cars as totally unacceptable because they are "generic" and have nasty plastic/rubber diaphragms. K-Line 21" cars are much better but just try tracking them down.