Dragon. But it won't really belong to NASA.

Great question!

IMHO, the cancellation of the space shuttle was the best thing NASA could have done, given the situation. It forced private industry to build real competitors to the shuttle, efforts that had dragged for years and years. Also, the space shuttle was always a much more expensive launch vehicle than NASA intended. The shuttle simply wasn't providing what the world needed: cheap access to Earth orbit.

NASA can now focus on long-range scientific projects that doesn't interest private industry, while industry builds a near-Earth space flight industry.