A quick google revealed this slideshow that says the nucleus is the easiest organelle to see. Generally, this is what is taught in schools worldwide. Note that mature red blood cells don't have a nucleus, so this isn't a universal rule. The visibility of the nucleus also changes throughout the cell cycle.
In case they are trying to trip you up, double check what the definition of an organelle is. The easiest cellular structure to see is the cytoskeleton by proxy of the cytoplasm. The cytoplasm forms the largest portion of the cell, it can be easily identified as the space between all the other organelles, and it is universally present in all cells. Indeed, many definitions of an organelle allow cytoplasm and all should allow the cytoskeleton.
any of a number of organized or specialized structures within a living cell. -Google
a structural and functional unit, such as a mitochondrion, in a cell or unicellular organism - Dictionary
However this is the kind of answer akin to "skin is the largest organ in the human body", and isn't what your homework is guiding you towards.