There many kinds of cells, sometimes asking for the 'largest' organelle does not make a lot of sense. Why? Because of the diversity of cells, especially in plants:
The largest relative volume of a single organelle (what
@Ygggdrasil answered ) is a way to provide a kind of answer. This varies across tissues in animals and plants as well.
Most times the answer is: Usually the largest single organelle in a cell is the nucleus. In both plant and animals.
If this if for HS biology, then go with what you have in the post above, but be aware of the weirdness in the world of cells.
Really there is no one perfect answer, you always find something goofy:
Multinucleate cells - one cell, many nuclei, relative volume of nuclear material is high. Does this count?
Fungal hyphae (Basidiomyces) - kind of like the root body of some fungi like mushrooms - are mutlinucleate, but also have huge amounts of ER.
Acetabularia is a GIGANTIC single cell (3 inches high sometimes), so the is the ER the largest? (These things are just plain wonderful).