Which word describes someone who is extremely perceptive?

The word that best describes someone who is extremely perceptive is "perspicacious." This term conveys a keen ability to notice and understand things that are not immediately obvious to others. A perspicacious person is not only observant but also able to interpret and analyze information effectively, making insightful conclusions based on their observations. This quality of being perceptive encompasses both an acute awareness of their surroundings and deep comprehension, allowing for a clear understanding of complex situations or subtle nuances.

In contrast, while "astute" also refers to someone who is sharp or clever, it often implies a shrewdness in practical matters rather than the deep insight that "perspicacious" embodies. The other words, "oblivious" and "inattentive," describe a lack of awareness or engagement with one's environment, which directly opposes the quality of being perceptive. Hence, "perspicacious" is the most fitting choice to describe someone who possesses extreme perceptiveness.