The phrase "most shackling" implies intense restriction or imprisonment, and there are several synonyms for conveying this sense of confinement. "Restrictive" suggests limitations on freedom of movement or action, while "constricting" implies pressure or tightening. "Oppressive" conveys a sense of overwhelming weight or burden, while "stifling" suggests suffocation or lack of air. "Incarcerating" specifically refers to imprisonment, while "enslaving" implies the loss of agency or autonomy. Ultimately, the choice of synonym will depend on the context and the degree of intensity desired in the description of the confinement.