N scale has improved dramatically over the last decade or so. All of the current products are vastly improved over their predecessors.
In terms of quality freight cars, the leader of the pack has to be Micro Trains (MT, formerly Kay-Dee). Their knuckle coupler system is the defacto standard in N scale. MT freight cars are well detailed and usually operate flawlessly. Be aware that MT offers low-profile wheels for their cars. Low-profile wheels have very small flanges and look more prototypical, but can be prone to derailments if the trackwork is not dead perfect.
MT produces a lot of limited-run "collector"-type cars. This often leads to products that are no longer available shortly after being introduced to the market. Great if you like to collect, not so great if running trains is your goal. Some of these are "specialty" cars, like their highly popular State Car series and their annual Christmas cars. These are really for collectors, as they represent no prototype cars.
Atlas has always produced excellent running freight cars of all types. Very good detail and reasonable prices. Most come equiped with Rapido-style couplers and/or Accumate knuckle couplers. The Accumates will couple with MT's, but will not uncouple with MT's magnetic uncouplers. Atlas cars are easy to convert to MT's.
Bachmann needs to be mentioned, if for no other reason than they are prolific in their production of freight cars. Some are better runners than others, all can be improved with a little work.
Intermountain makes very nice products, but I don't have much experience in running them.
There are a lot of older production cars still available, both new and used. Of the older production models, Con-Cor was perhaps the best. They were well detailed, easy to convert to MT's and had metal wheelsets. Metal wheelsets seem to operate more reliably than their plastic counterparts. Many modelers upgrade from plastic to metal wheels to improve operating characteristics. Be aware that different manufacturers often use different axle lengths, so if you are planning on upgrading, make sure the replacements are designed for the manufacturer's product you wish to upgrade.
There are other freight car manufacturers out there. Your hobby shop may have a good selection from different companies. Look them over carefully and, if possible, ask to put them on a length of track and see how easily they roll. Cars that roll easily allow you to pull more cars on a train.
Hope this information helps you. Don't hesitate to ask more questions as you get farther along in this hobby.
Darrell, quiet...for now