關於英文既問題

Est is used in comparison
e.g. A is 30kg, B is 60kg, C is 120kg
Then, C is the heaviest. (C是最重)

Extreme can be used to describe only 1 thing.
e.g. C is extremely heavy. (C極度非常重)
C非常重的事實, 不用比較也看得出

Firstly, i got to point out that -est is not really a comparison, it is the superlative of the adjective or adverb.
In my own opinion, extremely something is much more effective than adding -est at the end because the word extreme is quite a strong word and that-est is quite common and does not have the element of surprise within it, if you wanna make the word stand out.