A single food or a meal doesn’t matter
A single food then probably liver. But you don’t want to be eating liver every day.
A single meal then, a rib eye or some fatty meat, combined with lots of mixed veg. This has the benefit of fibre.
But this is a terrible way to approach a diet. What the brain “requires” is completely different to what it needs for optimal performance. You need a varied diet for optimal performance. You might actually want to limit red meat for optimal health and performance. So actually a food/meal which has everything you need, might never make it onto healthy diet.