For me it’s heels.
Due to my upbringing, I see “being feminine” as “being submissive or less in control”. I can do all my manly daily activities in more or less any type of clothing, but high heels change the way I walk, balance, carry myself, and when I go out they change the way I rely and trust people because I can’t run fast to escape a threat. So, nothing to me says “feminine” more than a pair if high heels.
Second position, for similar reasons, would be tight long skirts (think pencil skirts). Third position, still for a similar reason, are tight corsets.
The more constricting a garment is, the more feminine it makes me feel.