It uses SVG and has support for 90+ chart types, including 3D, gantt, funnel, various gauges and even maps of world/continents/countries/states. Also, most of the charts have both 2D and 3D versions.
They are very easy-to-create and, optionally, an official jQuery plugin exists to simplify it more. Just define the chart type, insert the data and you are almost there.
Charts are completely customizable. The labels, fonts, colors, borders, etc. can all be changed.
And, they are heavily interactive with tooltips, clickable legend keys, drill-down, zooming/scrolling and one-click chart export or print.
Considering that the charts are rendered in SVG, they are compatible with any desktop or mobile browser including the old versions like IE6.
When building something complicated, besides the features, the support becomes important too. FusionCharts Suite XT is very well-documented, has an active community forum and comes with pro support.
The library has a detailed pricing structure that covers many use-cases like: commercial website, intranet, Saas app and more.