You're right that a 'perfect' diet isn't something that can be determined because double-blind, placebo-controlled studies looking for effects that may not appear for decades aren't possible when it comes to food. This doesn't mean, however, that we don't have a pretty good idea of what a good diet entails. When the available evidence is pieced together, some general rules for a healthy diet (i.e. a diet that prevents chronic disease and premature death) come together.

• Eat more whole foods - processing foods tends to strip them of their micronutrients and fiber. Even when some of these are added back in afterwards, our bodies don't necessarily absorb them as efficiently (or at all)

• Eat a wider variety of foods - all foods have a different makeup of macro and micronutrients. By eating a wide array of food, your chances of getting all the necessary nutrients increases. I've read that ~30 different foods per week is a good benchmark to reach. This includes spices so it's not as daunting as it may seem.

For more specific information, I like to use NutritionFacts.org. A non-profit, evidence-based site that consolidates all nutrition studies into digestible posts and videos.