Reddit mentions of 30" Large 3-Levels Ferret Chinchilla Sugar Glider Rats Mice Mouse Rodent Hamster Gerbil Hedgehog Small Animal Critter Cage

The Critter Nation is pretty universally recommended. It's not the perfect choice for everyone but nothing's perfect.

Pros;

• Very large. The single unit is a mansion for 2-4 rats.
  
  
• It's super sturdy. The Critter Nation cages are basically huge pieces of wrought iron furniture.
  
  
• Double doors. The entire front of the cage opens up fully which makes accessing your rats and cleaning really easy.
  
  
• You can add to the cage if you get more rats or just want to spoil a pair of rats.
  
  Cons;
  
  
• Very large. If you live in a small home/apartment or if you keep your cage in your bedroom, the CN eats up a lot of space.
  
  
• Because it's so heavy, you clean it where it sits. You generally don't want to have to move it around very much. Also, all of the hollow metal components and tiny connection points can trap water and moisture and cause rust over time. You want to be careful with how you clean it.
  
  
• There's a lot of negative space in the cage. This means that you have to add a lot of accessories (hammocks, levels, etc.) to fully utilize the space. This can just add to the time it takes to clean the cage.
  
  
• The shallow plastic pans work well for fleece liners but notsomuch for a loose substrate like aspen, carefresh, etc. You can purchase deeper custom pans or you can trim a concrete mixing tub to fit the cage if you plan to use a loose bedding, though.
  
  If you live in the US, there are a few other decent, smaller cage options.
  
  Martin's Cages ($105-140): Martin's cages are available in many different sizes but I would recommend the R-680 (30w x 18d x 24h) and larger. If you think you might be keeping more than two rats at some time in the future, you may want to go with the R-695 (30w x 18d x 36h), but it does get pretty tall if it will be sitting on a desk. Martin's give you the option of purchasing their cages with just the bare, uncoated galvanized steel bars or with a powder coating on the metal. For rats, you'd always want to pay a little extra to have the bars powder coated. Bare, uncoated galvanized steel will absorb urine and odors over time and they're impossible to wash out. I have a Martin's cage and I absolutely love it.
  
  Large MCage ($60): The MCage has the same dimensions as the Martin's R-680 (30w x 18d x 24h) but it doesn't appear to be as high-quality. This is definitely a "budget" cage but for the money, I don't think it looks bad. It does require some modification. A wire mesh floor with a pull-out pan is not appropriate for rats. I've seen pictures of this cage with the wire floor removed and the pull-out pan locked into place and I think that would work better
  
  Rat Manor ($90):
  I think this is the most common cage for rats in the US. This cage is taller and narrower that the other cages (22w x 16d x 32h) so it doesn't take up as much desk space but the proportions seem a little cramped to me. Perhaps you don't have to use all of the levels. The cage might be better if you just use the half-levels or one half-level and the mid-level floor. The doors are tiny on this cage. A litter box wouldn't even fit through these doors so the whole cage would need to be taken apart every time you change the litter or clean the cage. I've seen this cage on sale online for $60 and that seems like a more appropriate price. I wouldn't pay $90 for it. If you can't find a good deal, I'd go with a Martin's cage instead.
  
  You can also search online classifieds for a used cage. Here are some recommendations for picking out a cage;
  
  
• Wire mesh/bars on at least three sides for adequate airflow.
  
• 1/2" spacing between the bars to prevent escapes. Ferret cages often have 7/8" to 1" spacing between the bars. Juvenile rats and even smaller adult female will be able to squeeze between these bars. You can purchase 1/2" hardware cloth to attach to the bars of the cage to prevent escapes but it's not terribly attractive and can make the cage more difficult to clean.
  
• Enough space for the number of rats. I'd start at around 6 ft^3 for a pair of rats and add an additional 2 - 2.5 ft^3 for each additional rat.
  
• No chewable components. Any plastic cage component should be out of the reach of rat teeth. Some cages have a plastic base that can be chewed through. "Drop in" plastic pans are fine, though.
  
• Doors that securely latch and are large enough for you to access all areas of the cage.
  
  As a preference, I like cages that have a fairly large base. Being that rats aren't arboreal creatures, I think running around space is a little more important than climbing space. If forced to choose between a wide, somewhat low cage and a skinny but tall cage, I'd go for the wide cage, but that's just a preference.