The word 'undermining' refers to something that weakens or erodes the foundation of something. Antonyms for the word 'most undermining' would therefore be words that convey strength, stability and support. Some possible antonyms include 'constructive', 'encouraging', 'uplifting', 'supportive', 'reinforcing', 'empowering', 'strengthening' and 'beneficial'. These words suggest actions or behaviors that contribute to the growth and development of something, rather than tearing it down. By incorporating positive antonyms into our vocabulary, we can promote a culture of positivity and inclusiveness, and work towards building a more supportive and productive society.