However, of the 302 appearances of 'bitterest', 134 are in 2016, while of the 441 appearances of 'most bitter' only 115 are in 2016. The difference may (or may not) reflect a recent trend favoring 'bitterest'.
If you're defining 'proper' and 'correct' in terms of rules, it comes down to what sounds better to you or your audience.
About the NOW corpus, the compilers say this:
The NOW corpus (News on the Web) contains about three billion words of data from web-based newspapers and magazines from 2010 to the present time. More importantly, the corpus grows by about 4-5 million words of data each day (from about 10,000 new articles), or about 130 million words each month.
With this corpus, you can see what is happening with the language this week -- not just 10 or 20 years ago. For example, see the frequency of words since 2010, as well as new words and phrases from the last few years.