I use a Giottos ball head (MH 1001). I find it very convenient compared to a pan tilt head when it comes to 35mm or 6x6cm. I have used it with lenses upto 300mm f/4. You may want a larger one if you use longer or more heavier lenses (the MH 3000 is really nice). When it comes to LF, I realized last weekend that I would be better off with my pan tilt head.

That said Arca Swiss make the best ball heads I have seen. What ever route you go, make sure you have the pan bed, tension lever and a locking lever.