The Arca Swiss head is expensive but it should last a lifetime and is a pleasure to use. So if you like ball heads it will serve you very well. It has a capacity of up to 90 lbs so no matter what format you use, from 35 mm to 8x10 or theoretically even larger, it will easily support your camera and lens and you don't have to go to a larger, heavier head every time you move up in format size. However, as others have said, I personally don't think a ball head works too well with a larger, heavier 4x5 and up camera. I used to have a Tachihara, which weighs under 4 lbs, and my Arca Swiss worked fine with it. I now have a Linhof Technika V, which with lens weighs around 7 lbs, and the ball head is marginal with that. As you get up in camera size and weight you just have so much size and mass moving around when you try to adjust the head that it's hard to set everything exactly as you would like it. I've started using a Bogen 3047 head with the Technika and I like it fine.

If your camera has a revolving back one thing to check before buying any head is the ability of the back to rotate without bumping into the head. With some heads (e.g. the Arca Swiss if situated so that the head is parallel to the back of the camera, and the Bogen 3037 head, which I also tried) it is impossible to rotate the back without it bumping into the head. If your camera has a reversible back (i.e. if you remove the back in order to switch from horizontal to vertical and vice versa) this isn't a problem.