a question of sherlock holmes

I remember Sherlock Holmes once said that the most beautiful woman he had ever met in his life is a murderer...

Can anyone tell me in which story he said this? Thanks

Hmm. I don't recall this thing you mention. From what I remember, Sherlock Holmes was only captivated by Irene Adler, a woman who "managed" to beat him intelectually, as a manner of speaking, as she escaped his investigations, albeit she chose to award him what he was seeking for. The tale I'm referring to is called A Scandal in Bohemia, and you can find that story in this site right here.
Hope this is of some use. :)