Which word best defines the opposite of 'sufficient'?

The word "insufficient" is the best choice for defining the opposite of "sufficient." The term "sufficient" refers to something that is enough or adequate to meet a requirement or need. When something is termed "insufficient," it indicates that it does not meet that necessary level or requirement, implying a lack or a shortfall.

In contrast, the other words, such as "ample," "plentiful," and "abundant," all convey a sense of being more than enough or exceeding a requirement. Therefore, these terms do not convey the idea of lacking or falling short, making "insufficient" the most fitting antonym for "sufficient."