Intuitive and Interactive. Those are two of the most important qualities that a visualization should have. A visualization needs to convey information in a way that makes sense to the person seeing it without having to explain it. On the other hand, it should also allow for further exploration and pique the person's interest to learn more.
As a quick exercise, I decided to do just that with a neat D3 visualization I recently came across. Referred to as "Zoomable Circle Packing", it provides a way of visually grouping together related data points. Each data point is represented by a bubble. Related bubbles are then encompassed by a larger bubble, and so on. Besides the fact that it is interactive, allowing someone to zoom-in to see more detail or zoom-out to look at the bigger picture, it supports an indefinite level of grouping. This makes it a very powerful visual for data that can be represented through a ragged hierarchy (where routes down the hierarchy can be of different lengths). Furthermore, "Zoomable Circle Packing" let's you incorporate weights for each one of the data points to see the relative value of each compared to the others in the group. The bigger the bubble, the bigger the value proportional to the others around it.
Below is the example I created of this D3 visualization. It uses data on some of our capabilities at Third I. Click in and out of bubbles to learn more about what we do (and how the visualization works, of course). Go ahead, try it out!