I love the Lionel Postwar F-3's and have enjoyed many of them for over 53 years in the hobby. For Postwar road names not available, I took basket cases and repainted them much to my liking. Budgets were tight in Postwar America at Christmas so Dad added something each year to keep up the tradition of Lionel trains @Christmas.
Self Propelled engines became well liked because they were "different" and doable price wise.. .
So my nod goes to the "self propelled" units- especially the gang car. It took a lot of abuse for years (flat out as fast as you could run it) till the gears gave out. Ours did! Little boys liked them because they were the fastest things you could put on a track when we had the "need for speed". They were also easy to put on the track with little hands..
Others units didn't survive as well because of flimsy window struts( heck that's where a kid picked them by). Survivors in nice condition are so uncommon and desireable!
Glenn Spencer