Cage choice seems to be a very individual thing, but there are certain points you need to bear in mind.
Your rats' cage needs to be big enough to allow them to express their natural behaviour. The minimum I would recommend for a pair of rats is 80cm x 50cm x 50cm, as they not only need space to live and play, but also space for cage furniture. Within reason, the bigger the better, although nervous rats may benefit from a smaller space to begin with, while they get used to interacting with you.
The gap between the bars is important too. Some really nice looking cages have wide gaps that will let young rats and small does walk straight out. Look for bars that are not more than 12mm apart, preferably a little closer for young rats. Powder coated bars seem to keep their looks better than chromed bars, which discolour from contact with urine. Dark coloured bars are easier to watch your rats through; white bars in particular seem to distract the eye. Also some rats will chew plastic bases.
Another consideration is ease of access. If your rats are a little timid, you need to be able to reach them. If the cage is difficult to clean out, you're not going to want to do it. Look for a cage with a nice big door which is positioned to allow you access to all corners of the cage and which will be easy to hang hammocks and ropes across. If you like to tip out the substrate straight into the bin, look for a base which will fit easily through any doors.
I've used Amazon to provide the images below, so if you do happen to click through to buy I will get commission. Please check out other vendors before you buy anything, as prices do vary and you can sometimes get a much better bargain.
My personal preferred cage is the Liberta Explorer, a tall cage which is 90cm wide, 60cm deep and 120cm high, set upon a stand and with large doors that open to the full width of the cage. I use this as an open space without a level halfway down, but with hammocks and ropes to break any falls, and I use custom made metal shelves in the base to replace the shallow plastic trays which come as standard. They're usually available at a fair price on Amazon, but it's worth checking eBay for second hand ones in your area.
Liberta Explorer on Amazon
This is a similar sized cage to the Explorer, but more solid (and expensive).
Savic Royal Suite on Amazon
These three cages are all plastic based and a lovely size for a two to four rats. Click on the pictures for links to the Amazon information pages.
The Abode is another very popular cage, although the base cage is very shallow so you may eventually want to replace the tray with something deeper.
Liberta Abode on Amazon
This is a nice sized and sturdy cage that has the advantage of being flat packable. It's difficult to fix hammocks into the corners, but nonetheless is fairly popular and has a good sized door.
Ferplast Furet Plus on Amazon
This one I'm not too keen on, but I am aware that some people love it, so it makes the list. The doors benefit from being taken off and turned round so they open outwards rather than inwards, and the mesh shelves need either covering or taking out. It wins on price, being less expensive than most other options of a similar size.
Chinchilla 3 on Amazon