What is the proper superlative of bitter? Is it most bitter or bitterest?
I am assuming that either of these is the correct answer and I cannot recall hearing one more often than the other, and bitterest sounds strange to me despite the fact that it may be the correct option.
Both forms are well represented, with bitterest being the more common of the two as this ngram illustrates:
Superlatives of two-syllable words can be formed either by inflection (adding -est) or by using the most. Some are more common than others, depends on the syllable structure.
These two are not mutually exclusive options, and both can co-exist happily, as we see in a most bitter conclusion side by side with his bitterest enemy. Both are correct, and the choice depends on the writer or speaker.