Preferably grain-free, although there are some decent wet foods that do have a small amount of grain like brown rice (Precise comes to mind). Stay away from the really cheap grocery store foods like Friskies or 9-Lives because they are heavy on the un-named (eg "meat") by-products and food colouring. Try not to feed too much fish because it's higher in toxins and is a common source of allergies for cats. Depending on your cats, you may want to rotate a few different flavours and brands to keep them interested (some cats get bored of the same canned food every day).
As for how much to feed, depends on what your cats weigh, and whether they need to gain or lose weight. Usually, a 5.5oz can each per day is a good benchmark. If they're still really hungry, feed a bit more, if they leave lots behind, maybe they need less, although make sure they're not just being super finicky. They still need to get enough calories to avoid other medical problems.
Here is a good article on how to choose a canned food:
http://www.catinfo.org/commercialcannedfoods.htm