The phrase "most stupefactive" refers to something that is extremely numbing or dulling to the senses. Other possible synonyms for this phrase might include "highly soporific," "incredibly somnolent," "very dulling," or "exceedingly lethargic." Each of these terms describes something that causes lethargy, sleepiness, or a lack of focus. Other possible synonyms might include "supremely narcotic," "incredibly mind-numbing," "truly stupefying," or "utterly numbing." Ultimately, the choice of synonym will depend on the context in which the word is being used and the specific connotations one wishes to convey.