I think if you compare Rear Window to the rest of the Alfred Hitchcock catalog, it stands out for making the most of a small number of moving pieces. Hitchcock made almost exclusively suspense thrillers and most of them are
very plot-heavy. In contrast, Rear Window is set in one location and even more significantly its limited to a large extent to the same view out the window of that room. It generates its various highs and lows mostly from character revelation and exploitation of that limited point of view. As the audience we only have access to the same information as the main character, there's no chases or action set pieces, there's no theatrical lighting or exotic locales, and yet it lacks nothing in suspense or entertainment for all those restrictions.
That's why for a large number of people Rear Window is the ultimate Hitchcock accomplishment. Its parts fit together like a swiss watch, nothing is wasted, everything has a place. It's not my favorite Hitchcock movie, I think we talked about this in the last movie draft -- for me that's Vertigo. But I certainly understand why other people would put Rear Window at the top. And that "relative minimalism"
@Löwenherz mentioned is a big part of why it's such an impressive piece of work to so many people.