The phrase "most rip-roaring" refers to something that is exciting, lively, and full of energy. Its antonyms would be words that describe the opposite characteristics. Some of the antonyms for "most rip-roaring" are dull, lifeless, boring, uneventful, and lackluster. If something is dull, it lacks excitement, and if something is lifeless, it lacks energy. Boring and uneventful refer to things that are not interesting or exciting. Lackluster means lacking in enthusiasm, energy or brilliance. Therefore, the antonyms for "most rip-roaring" are words that describe things that lack the liveliness, excitement, and enthusiasm that the original term conveys.