Unequivocally, the best sandwich in the Pokemon and real world universe is the Ultra Peanut Butter Sandwich. It contains banana, peanut butter, butter, and, optionally, jam.
In the real world, the spicy herba mystica is also an amazing addition, but I like the egg power in the games from the Great and Ultra level sandwiches.
recipe
crusty baguette
sunflower butter
crunchy peanut butter
spicy jam
a banana
Cut your baguette longways through the side
Apply butter to bottom slice, and peanut butter to the other
Cut a banana into thick slices and layer them onto the butter side