What is a word to describe selecting the absolute best word to use in a sentence? This is a problem for writers.
Mot juste means the exact, appropriate word.
The exact, appropriate word
If it's the process of choosing words, irrespective of them being good or bad, call it "wording", or "phrasing". Definitions from M-W:
the way in which something is said or written : the words that are used to say something
the way something is expressed in words : the particular words or the order of words that are used to express something