It's hard for me to pinpoint what technique is hard for me to play...but I'm constantly always on the search for classical music and playing it on the electric guitar because to me that type of music uses the most challenging of many different techniques combined and to me the most beautifull music ever created.

I would say that for me what is harder than any technique would be phrasing..something that I'm always striving to perfect.

Another thing that I'm always striving to do is to perfect the tone in my hands.

I guess if I had to pick the single most difficult technique to perfect would have to be vibrato.

I'm also a sweep whore :-p