Buff!

I can only speak for myself, but I don't think one is more effective than the other; whether fully clothed or not, photos should be "classy" (as opposed to raw and trashy.) I don't know the formal distinction between "sexy" and "sensual", but what appeals the most to me is when a woman's sensuality comes through in her pictures. She doesn't have to be totally in the buff. In fact, some of the photos that are the most exciting to me are the ones that hint at more to come.  

I am a serious (but amateur) photography and always try to learn from excellent photographer's work by studying their use of lighting and how they use it and depth of field to focus the viewer's eye. I believe that is what most people would say is distinctive about a well-done photo.  But I really am not a fan of pictures with heavy-handed post processing (e.g., Photoshop). A little goes a long way.  Some ladies have very professionally done photos that are so "photoshopped" that their skin doesn't even look real. Again, only speaking for myself, I think that is distracting and it gives a feel that something is being covered up and hidden.  

Kind of long-winded, but the short answer is a "well-done" photo, whether fully naked, partially clothed, or completely clothed, can be extremely sensual and inviting.

can be simply beautiful:

I really like it when a lady wears a sexy outfit when greeting me .

tell you what sexy is....I just know it when I see it.