Rice and beans are "essential" only because they are a cheap source of calories. That does not make them optimal human food.Some cultures see almonds almost as divine food. Same with rice and beans, these are essential foods for 3/4 of the world.
As far as the O3/O6 tradeoff plus the anti nutrient factor, mac nuts are the best (Mark did an article all about nuts.) Almonds, IIRC, were a close second.