I tested AGG, Cairo, GDI+ and Quartz (for Mac).
I think Quartz is the best, but is available (as long as I know) for Mac only.
AGG is poweful, but is not well documented. The developer decided to reinvent the wheel, and made his own doc system, instead of using something standard like doxygen. There are good tutorials for basic understanding, but when you dig deeper you find API documentation lacking, imprecise or incomplete.
GDI+ is pretty basic compared to the others, and is available for Windows only.
As a result, I think the best choice is probably Cairo (unless you can choose to develop for Mac only). It's well documented, the code is clean, and is fast and powerful.