In the comics it is written on Mjolnir that "whosoever is worthy to wield the hammer will have the power of Thor". If so who in the Marvel Cinematic Universe is really worthy of gaining the hammer and should Thor be defeated to lose the worthiness?
So far, on-screen, there are only two-ish people, that have been shown to be worthy to wield Mjolnir.
- Thor, obviously, is the primary wielder, and has been since he restored his worthiness in Thor
- The Vision was shown to be worthy to wield it in Age of Ultron, picking the hammer up with ease. There's debate over if this counts, since he's technically a robot, but he's able to use the hammer as a weapon so it's close enough.
- There was a hint in Age of Ultron that Steve Rogers might be worthy; he managed to shift Mjolnir in place -- something no one else could do, and something that worried Thor -- but ultimately didn't actually lift it.
In the comics, there's a whole list of people (see, e.g. this scifi.SE question) who were worthy at one time or another, but so far we've seen no evidence that any of those people will be capable of wielding Mjolnir in the MCU.