This question will use The Economist's Democracy Index, resolving YES to the one country whose overall democracy score difference between 2029 and 2019 is most negative (i.e. the 2029 score minus the 2019 score).

As of the 2023 report, the five countries that have backslide into authoritarianism the most in the 2020s so far are (i.e. 2023 scores compared to 2019 scores):

1. Myanmar (-2.7)

2. Afghanistan (-2.59)

3. Mali (-2.34)

4. Haiti (-1.76)

5. El Salvador (-1.44)

For reference, here are the countries that backslide into authoritarianism the most for previous 10-year periods of the Democracy Index, which began in 2006 (all data taken from Wikipedia's table of Democracy Index scores):

Feel free to add answers that are covered by the Democracy Index.