Please tell me if I need to use the definite article in the following sentence.
What I want (the) least is to work there.
I feel that I need to use the there. If I am right, please tell me why it should be there.
Least is an adverb in your sentence. It means we can use the, or we can omit it. If least were the superlative adjective, we would have to use the. Compare:
He is the best person I know. ("best" is the superlative form of "good").
I like this dress best/the best. ("best" is an adverb)
At the least sign of trouble, he gets scared and quits. ("least" is the superlative form of "little")
The man who actually won the car is the one who wanted it the least/least. ("least" is an adverb)