"Only X can save the world/stop the bad guys/catch the killer": who doesn't love that angle?
I'm sure most of us immediately think of the mystical Chosen One trope (Buffy being the prime example) but that's not the whole picture. I'd argue that situations sometimes choose a hero, not just whatever Power is seen as pulling the strings. Think Die Hard, for example - circumstances focus to a point that forces him (because, you could argue, of his innate nobility) to take action and Save the Day. The situation makes him 'that guy', but we know that any other guy wouldn't (couldn't) do what he does.
Many detective stories operate in this way, I think - especially where our hero is seen as making sacrifices for his/her calling. This adds to the nobility of the situation. I know some might claim I'm stretching this trope to breaking point, but I'd want to include teams here too, particularly the 'greater than the sum of their parts' type. I'm thinking of shows like Criminal Minds, where skills are shared around the group, but all are clearly needed to achieve their exemplary results.
Isn't Hero's Journey stuff fun? :)