That's really not a question I feel qualified to answer - after all, I haven't tried all the surgical subspecialties so any answer I would give would be largely based on anecdotes I've heard.
However, I will advocate Plastics and Reconstructive Surgery (PRS) as being highly versatile, with a grand variety of procedures, on all ages and all parts of the body. Like any specialty though - surgical or not - you may find yourself doing more and more specialized work as you become known for a certain procedure. Generally, the more you do something the better you become at it.
At any rate, PRS is something that excites me but I'm sure there are other surgical fields with lots of variety as well.