My initial reaction was the power that Rouge has in X-men because cricky, the tought of one single touch all but and maybe even killing someone I know and love is a rather horrible one.
I'm noticing a lot of 'no midas touch please' comments as well, and so I think the worst sort of superpower to have is one you can't 'turn off', so as cool as being Nightcrawler is, I'd hate to be blue and demonic all the time, same goes for Mysteque I guess. And as groovy as it would be to have laser vision, if you couldn't turn it off.... oh dear.
Oh, and for all those who say telepathy would drive them mad, all I can say is, Charles Xavier, coolest guy ever.
(Why yes, I watched X-men: First class yesterday how could you tell with all this X-men referencing I'm doing?)