Since this thread has been dug up and replied to, I am going to respond to it now. The best diets aren't the common mixes for rats that you can find in the stores.
There are lab blocks such as Harlan Teklad which provide complete nutrition that are better than seed/nut mixes. There is Suebee's mix which supplemented with certain brands of dog foods such as Innova Senior that make good diets. Also you want to supplement with fresh veggies, occasional fruits, they can have dried fruits as well in moderation. Plus there are many other foods they can have.
I personally don't like the store bought mixes because I feel they don't provide complete nutrition. They tend to be full of nuts which are high in fats, corn products, seeds and other stuff.
Here is a link to Suebee's mix.
ratsrule.com - suebee's rat diet
I use this myself along with the Harlan Teklad blocks, fresh veggies, fresh and dried fruits, cooked whole grain pastas and rice, a bit of meat 2-3 times per week (usually chicken, tuna or other fish)and other treats now and again. Other treats include cooked oatmeal, yogurt, dark chocolate M&M's, bits of bread soaked in olive oil, and bits of things off my dinner plate from time to time (healthy things).