Having acquired a stellar small tripod last fall (Mo’ Gitzo) I subsequently needed to outfit these legs with a small-ish ball head. What is small-ish? It should weigh no more than 1 lb and should support a camera (with lens) of 20 lbs.
Because I equip my cameras with a quick-release system that called “Arca-Swiss compatible”, I immediately eliminate any possible choices that will not support Arca-Swiss quick-release. (So, for example, I rule-out some tripod heads by Manfrotto, Giotto, Benro, etc.)
Markins Q3T ball head is designed with a diameter that is perfect match for a Gitzo GT1541T tripod. However, while the Q3T is ideally suited for the GT1541T, it offers no particular advantage when paired with the GT1542T, which I have. (If you are taller than 5’7, the GT1542T is a better choice than the shorter GT1541T
Some other possible choices include Really Right Stuff BH-30, Fiesol BC-40D, and Induro BHL1. But, in the end, I chose the PhotoClam PC-36NS. The size is perfectly suited for the GT1542T and it only weighs just a bit more than 12 oz (350g). The variable friction is extremely smooth and requires no break-in period. Includes a built-in bubble level. $209 from reallybigcameras.com.
Shown in the photo above: Linhof Profi III (big, very expensive, smooth like butta) with a Kirk Enterprises clamp, Arca-Swiss Z1 (the standard in professional ball heads), PhotoClam PC-36NS.