We used the best stopper we could find.[good changed to superlative form best with -est]
We used the smallest stopper we could find.
[small changed to superlative form smallest with -est]
We used the most tapered stopper we could find.
[superlative with most]
We used the least tapered stopper we could find.
[superlative with least]
If you are comparing only two items, use the comparative form. If you are comparing more than two items, use the superlative form.
This is the smaller of the two stoppers.[comparative]
This is the smallest of the three
If you use a comparative adjective, make sure that your comparison is complete by including the standard of comparison where it is relevant.
This stopper is smaller than that one.[standard of comparison]
If you are not sure how to form the comparative or superlative degree of a particular adjective, check the adjective in a dictionary.