They're both correct, but they mean different things. 1. She works the hardest in the office. This means that she works harder than anyone else in the office: she works harder than all her colleagues. 2. She works hardest in the office. This means that she works harder in the office than anywhere else: she works harder when she is in the office than in any other place where she works.

"She works THE HARDEST in the office" is the most natural because using "the" is needed for referring to a comparison between her and the other people in the office.

She works the hardest in the office.