I'm going with the classic: Jedi. They duel, they mind trick, they pull star destroyers out of the sky. They manipulate entire political systems to their whims. They're...filled with midichlorians (OK, we can pretend we never heard that one).
I haven't played Thief or Halo so I can't comment on their respective options.
Grey Wardens go through a deadly joining ritual so that they can hunt evil beings who come from underground to destroy all life. They have the ability to end a Blight. That's the extent of their power, though. Pretty badass, but no Jedi.
The Brotherhood of Steel are just fallout soldiers. Whats the big deal?
Spectres are more political tools. They get a pass to go over the law. Being a spectre doesn't make them a badass. They were already badass and can still be one with or without Spectre status.