Final answer:
The word that should not be used in the comparative is 'more unique'.
Step-by-step explanation:
The word that should not be used in the comparative is more unique.
While faster, taller, and smarter can all be used in the comparative form, the word unique is an absolute adjective. It means that there is only one of its kind, and it cannot be compared to something else using the comparative form.
For example, you can say 'This car is faster than that car' or 'He is taller than his brother.' However, you cannot say 'This painting is more unique than that painting' because uniqueness cannot be measured or graded on a scale.