It's true: "depends on available light and how much DOF you want" as ShutterBug said. So actually there isn't a "best" aperture for this lens (nor any other one). Because of the optics it has, sharpness changes a lot in this lens. You should take a look at this chart done by DPreview in their review. They tested the lens sharpness according to the aperture and focal length, which will help you understand the characteristics of your lens.
I have the same and what I do is try to think about the focus point and the focal distance I need for the photo, then decide the aperture according to that chart. If you shoot in Av you won't have to worry about the speed but I'd suggest you to try your own combinations and not leave everything to the processor.