Based on my usage for homemade sheaths, and research: Good quaility cowhide is the best overall leather to use for these types of items. This catagory would include steer- and bull- hides, but those would probably be too thick for a watchband (at least the ones ive seen). Sheep and lambskin - "napa" leathers - are too soft and don't wear well. Beware "buffalo" hide, as it is usually cheaper water buffalo hide from Asia. I hear that american buffalo hide (really Bisonhide) is strong, but I've not worked with it, and it is also relatively think...like the bullhide/steerhide. Some goatskin is tough, but alot is cheap (from Asia), and good stuff is stiffer than good cowhide. Croc/aligator/caymen and some lizard hide are strong, but hard to get. Some (most?) lizard and snake skins are too weak and/or hard to work for sheaths....or watchbands, I think. Stay away from pigskin and bonded leathers....just too weak/cheap. I hear great things about kangaroo leather, but have never used it. I know it's illegal in California....not sure why.
So, I would recommend cowhide, overall. Croc is fancy, but much more expensive and doesn't breath as well as cowhide. Be sure to get good quality full grain leather.