Widescreen if it was made that way. But I am offended when I come across a film or TV show produced in 4:3 that has been artificially rendered as widescreen (or worse, letterboxed). That's just as bad as pan-n-scanning a widescreen film to 4:3. Anyone who wants to see a widescreen rendering of Citizen Kane or Metropolis is pissing on the graves of Orson Welles and Fritz Lang, IMO.
@JD Yeah you have to watch when you buy at Wal-Mart. It's one of the only places that still sells both widescreen and 4:3 versions of recent movies. But the format should be clearly marked on the back of the box. Similarly, if you're buying, say, the first season of Kung Fu from the 1970s or some movie from 1939 and the box indicates 16:9 letter boxing, then you know something is amiss.