This is quite an intuitive topic, I like it; I believe I am in the right frame of mind currently to answer this the way I should, as opposed to half-assing it.

If I am in a relationship with a person that I am in love with, my "secret weapon", my "hot button" would have to be the most erotic memory or position or manuever that me and the person I am with have experienced. Or perhaps a fantasy of mine that involves her.

When I am single and have no particular focus, I would have to agree with many of the male members in saying that it would be imagining being another females "secret weapon." I can think of no greater turn on.