I'll second the votes for the #50 Gang Car. First time I saw one was in 1957, at one of our two local authorized Lionel dealers. It was running on Super-O track, which was new that year. But I didn't actually have one of my own until about 1973, when it was my first Lionel purchase after getting back into the 3-rail world. Now I've got two of them, one from 1954 and another late-production model from the early Sixties.
My second favorite would be the #60 Trolley. I saw one of those at the same time I was being introduced to the Gang Car, and liked it at once. But again, I didn't actually own one till the mid-seventies.
Santa brought me a #41 US Army turbine switcher in 1957, and he probably got it at the same local dealer where I saw the Gang Car and the Trolley. It still runs and looks as good as it did that Christmas morning.
In the mid-seventies, I also bought a #53 Rio Grande Snowplow, NIB, from Madison Hardware. And yes, it has the "a" in the correct position. I've long admired the #58 GN Rotary Snowplow, but they've always been too pricey for my budget.
I also like the #54 Tie-Jector car, bur for some reason I've never bought one. I may have to change that at the next train show (not till November, alas).