Smell and taste are the only possible answers. Anything else and you're fucked for life. Sight and touch are obvious but as someone who takes care of an elderly parent with hearing loss, you do not want to lose that or skimp on the best damn hearing aids you can afford. Hearing is critical for interacting and understanding the world around you, and hearing loss is highly correlated with dementia and Alzheimer's. If you can't hear what's going on, you tend to zone out and ignore things and your mind degrades. Communicating with others also becomes strained and that exacerbates the mental decline.