Have to go with Atlas with their PS1 box cars and their 90 ton hoppers. Dated to be sure but still presentable and still commands a decent price on auction sites. As with all Atlas cars, if you dress them up with MT trucks/couplers and replace the plastic truck king pins with 2-56 3/16" screws you have a solid runner.
Anything MicroTrains or former Kadee. Everything else has priced itself out of my budget. I have a number of N Scale Kadee flats in various road names with body mounted couplers that out perform anyother flat car on the layout.
R8R, These N scale cars served the logging portions of one of my layouts. Crew car, tank car, and explosives car. Thanks for the compliment. Here's one more for you. Jim
Here's one of the ones I'm talking about: what I particularly like about these is that they're cast metal. Combine that with many of mine being loaded with metal M1 tanks and the weight quickly requires multiple unit power.
Like the Trainworks 85' flat cars. Also like Athearn bi-level auto racks. Hope the tri-level racks are as nice.