I've been asking myself this question since I got my first ferret over two years ago. I'm always reading, researching, and trying to find the best food out there for my ferrets to keep them as healthy as possible. I have Marshall and Path Valley ferrets, so in the beginning they were being fed a combination of Marshall and PV kibbles. I then switched to Zupreem, one of the Ferretstore.com brand of foods (which is now Drs. Foster & Smith), and Innova EVO feline (ferret version wasn't available around here). I then started looking into grain-free, high protein, low carb foods and came up with a bunch of cat foods on http://www.petfooddirect.com I also second http://www.stinkweasels.com It has a good variety of healthy ferret foods. My ferrets are now being fed Innova EVO ferret, Nature's Variety Instinct cat food, ZiwiPeak cat cuisines, Stella & Chewy's freeze-dried steaks and the Carnivore Crunch, AFS freeze-dried sprinkles, Wysong foods, and Zupreem (this is my least favorite of the bunch, but it's one of the better, easier to find ferret foods on the market, so I like to keep it in their mix just in case). I'd really like to get into feeding my ferrets a natural, raw diet, but I think the foods I'm giving my ferrets now are a good second option. Most raw feeders still give their ferrets kibbles on the side and supplement their ferrets' raw diet with some of the above things I mentioned. My basic rules when looking for foods - they should be grain free, low in carbs, high in meat protein, and I also try to stay away from foods with corn, tapioca, and potato, all of which are high on the glycemic index (ferrets are prone to pancreatic problems, so lower GI is better). However, some of the kibbles my ferrets get contain either potato or tapioca (Nature's Variety has tapioca, EVO contains potato), which is why I like to feed mine a mix. I would also recommend Natural Gold ferret food (I don't recommend using this as the main food source though, because the fiber count is a bit high) and Horizon Legacy cat food.

There's really a lot to know about ferret nutrition, and it's going to be hard to give them that perfect diet. Check out some of the Yahoo ferret groups and forums I have listed on my profile, they all helped me a lot and that's where most of my food info has come from.