After trying a TON of foods with Bandit (Friskies, Fancy Feast Classics, Wellness Turkey & Chicken Flavors, EVO, Nature variety's Instinct, etc.), the food we have finally settled on for being the best diet for Bandit is
Merrick's--I feed the flavors Cowboy Cookout, Surf and Turf, and Grammy's Pot Pie (and occasionally Turducken). I feed those flavors because the have the best Protein/Fat/Carb ratio, but if you decide to feed a different flavor make sure you're checking the carb content.
The reason why Bandit does the best on these foods is because they use high quality meat, are very high in protein, low in fat, with appropriate carb levels for a diabetic. They are also all low-glycemic foods (some foods, like Friskies, can have reasonable carb levels but have a higher glycemic index because they contain rice or other high glycemic ingredients--this can have a negative impact some (but not all) diabetics). Bandit had GI issues with some of the foods that have a high fat content (like EVO). He also used to be obese, so I've struggled with maintaining him at a healthy weight because of that. Obese cats have slower metabolisms, and they need a lot of protein to help reset their metabolisms. I had trouble maintaining Bandit at a healthy weight with the other foods--but the high protein content and lower fat of the Merrick's has helped him enormously. It even changed his muscle tone--he used to have a very soft body and now he's solid muscle.
I've also found the Merricks to also be very palatable to my cats. It was one of the only things my other cat would continue to eat in the final stages of her stomach cancer, when she got incredibly picky with food. Not every cat likes it, but several of my friends have had good success with their picky cats liking at least one of the flavors.
It can be price if you get it at a chain store like PetCo, but if you enter your zip code into the store finder you'll likely find an independently owned pet/farm supply store near you that sells it, and they usually have much cheaper prices and offer case discounts or specials. I'm actually paying less to feed the better quality Merricks than I was when he was eating Fancy Feast. Currently, Amazon has been doing a continous promotion on Merricks 5.5 oz cans where you get $5 off the cost of a case at checkout, so it's cheaper to order the food that way, too ($1.20 per can)--if you have tried it out and know your cat will eat it. Normally, prices on Amazon are about the same as PetCo ($1.40 per can).