I've used a few ball heads over the years. My favorite is the Arca Swiss. They're strong, smooth, and they have a very good quick release system. I use quick release plates from Really Right Stuff for it. These plates are specifically machined for your camera. Thus while strong, they are very unobtrusive; and the camera won't spin on the plate! The Arca's elliptical ball head is nice: As the camera moves away from the "home" position, the ball gives more resistence.