There are several synonyms for the phrase 'most slackening' that can be used to convey a similar meaning. These include 'largest easing', 'greatest reduction', 'most decrease', 'biggest slowdown', 'maximum relaxation', 'highest loosening' and 'utmost deceleration'. Each of these phrases indicates a significant decrease in intensity, speed or pressure, highlighting a moment of relaxation or slackening. Depending on the context, one of these synonyms may be more appropriate than the others. Regardless, they all emphasize a moment when things slow down, making way for relaxation and reflection.