I've been researching rat food for a long time now. The one I think is the best is Oxbow Regal rat but the problem is it's not highly palatable and most rats tend to stop liking it. The next best food in my opinion is Harlan Teklad 2014. And the problem there, it's not readily available to the general public.
The main thing to look at is your protein and fat levels, you want your protein as low as you can, 14% is excellent. Fat should be the lowest as well, no higher than 5%.
The ingredients are important as well. Most people are scared when they see corn as part of the first main ingredients. But corn is a carb and a good energy source with some nutritional value, not much but some and it's a pretty good filler. I don't like any block with soybean meal/oil. Soybean add phytoestrogen to the diet and in turn may or may not play a roll in mammary tumours and pituitary tumours but I'm not taking any chances.
I also don't like a food filled with preservatives especially BHA or BHT, ingredients also found in motor oil.
If I were to give a recommendation, I would tell you to give a mix of Oxbow and HT 2014. But if those are not available, then I would recommend the Living World Nutriblock sold at most Pet Valu. If you can't find those, then I would go for Hagan Extrusion for hamsters, which is formulated well for rats.