@Arnold D. Cribari - I agree with you. I love the cars that make noise. At first, I was like who would want more noise? Aren't the engines "loud" enough? I started with the NYC Vision line passenger cars. Those were on blow-out. I then got a set of 3 UP/SP cars, and those are great. The squeaks are fantastic. Yesterday FedEx delivered the Armour stockyard 3 pack to me. I unpacked them last night but didn't run them yet. Today will be a good day for that since it snowed here in Chicago.
To answer the question - My favorite, Lionel Southern 6565 wave boxcar. It's just a pretty car. I am also a big big fan of the Atlas 40' billboard reefers. All of them. No one of them in particular stands out as THE car, but I have about 30 that I love to pull in a long steam led freight.
Nice topic.
John