The #1 for me is the underhand serve. I know it can be seen as disrespectful, but if you had a big or effective serve like Fed, it would just be another way to keep your opponent on their toes. I think it should be considered a legitimate shot.
But the one I really wanted to talk about happened last night.(Here: http://www.youtube.com/watch?v=7d71sBNQmSg#t=1m57s
It is a drop-shot I guess just because it is hit softly and short -- but to me it looks completely different than a drop shot because it appears to be a full swing/follow-through, just hit very very weakly. Is this taught to people? If so, I rarely ever see it used, but it seems very effective in the right situations. For example, I think Tomic used it in a perfect spot in that video. He had the whole open court but didn't seem to be able to get a read on which way Federer was leaning, so he hit it weakly because Fed was a few meters behind the baseline.
Thoughts? Anymore shots that are useful but seldom used?