I am using metrichgraphics[1], it's based on D3.js
I was looking at charts.js but it didn't had enough options.
that being said, YMMV, horses for courses, there'a a LOT of different ways to chart and different tastes, but that's my take.
I like it. It took a little poking at to get what I wanted (stacked bar charts with some initially unselected), but I got it and its been working well. I came from a highcharts shop, and the setup there was different enough.
Easy to implement basic charts like line, bar, pie, scatter, etc. and extensible with D3.