Highchart has been so good for our work we HAD to pay for it. We've looked at almost every single charting library (mostly open source) but in the end, the power & flexibility of Highcharts coud not be beaten.

Another awesome thing is their support. Ask a question on their forum or SO, then you'll most likely get an answer with a working JSFiddle example either from the community or the developers of Highcharts.

This is #1 on our JS library list.