The answer to this problem is crayfish
Cells are not simple structures at all. They have numerous organelles and serve numerous functions working together with millions of other cells. They are highly complex machines on their own that are combined with other highly complex machines to build anything, from smallest to largest organisms. For some unicellular ones they even do other things that cells in multicellular organisms don't.
The most probable and the most likely answer that you would get in asking this question would be domain.
As for stating it as a taxonomic group, it is more appropriate to refer to it as category, but there is no arguing that both words can be interchanged so just take that as a personal preference. Domain would be referred to in modern taxonomic category as the category that includes the kingdoms and all the other levels of the taxonomic hierarchy.